A. Introduction

1 Boundaries are, still in the interconnected world of today, ‘the persistent and the preferred paraphernalia of international political life’ (Brownlie [1993] 137, 138). It is no surprise that the delimitation of the territorial scope of States by means of boundaries remains of great significance in a legal system which has made the ‘principle of the exclusive competence of the State in regard to its own territory … the point of departure in settling most questions that concern international relations’ (Island of Palmas [Netherlands/United States] [1928] 838; Palmas Island Arbitration).

2 International law nevertheless does not impose on States a requirement of possessing defined boundaries. Nor is there a requirement that States must have defined boundaries in order to be considered subjects of international law. In Deutsche Continental Gas-Gesellschaft v Polish State, the German-Polish Mixed Arbitral Tribunal (Versailles Treaty) highlighted that:

Whatever may be the importance of the delimitation of boundaries, one cannot go so far as to maintain that as long as this delimitation has not been legally effected the State in question cannot be considered as having any territory whatever. The practice of international law and historical precedents point to the contrary. In order to say that a State exists and can be recognised as such … it is enough that this territory has a sufficient consistency, even though its boundaries have not yet been accurately delimited, and that the State actually exercises independent public authority over that territory. There are numerous examples of cases in which States have existed without their statehood being called into doubt’ ([1929] 5 Annual Digest of Public International Law Cases 14–15).

More generally, uncertainty as to boundaries does not affect territorial rights and there is ‘no rule that the land frontiers of a State must be fully delimited and defined’ (North Sea Continental Shelf Cases [Federal Republic of Germany/Denmark; Federal Republic of Germany/Netherlands] [1969] at para. 46; North Sea Continental Shelf Cases).

3 The present entry is structured in the following way. After making certain terminological clarifications (see Section B), it sets out the various steps of the boundary delimitation process (Section C) before examining how States acquire boundaries (Section D). Subsequently, it turns to the judicial settlement of boundary disputes (Section E) and, finally, key aspects of the international law regime applicable to boundaries (Section F).

B. Terminology

4 As a boundary is a line, it will necessarily be a ‘breadthless length’ (Euclid’s Elements Book I Definition 2). A boundary refers in legal terminology to the line delimiting the territory placed under the sovereignty of a State. Thus in Frontier Dispute (Benin/Niger) (2005), the Chamber of the International Court of Justice (ICJ) referred to ‘the general theory that a boundary represents the line of separation between areas of State sovereignty, not only on the earth’s surface but also in the subsoil and in the superjacent column of air’ (para. 124 [Frontier Dispute Case (Benin/Niger)]; see also Beagle Channel [Argentina v Chile] [1977] at 131; Beagle Channel Dispute).

5 Two terminological clarifications are necessary in this regard. First, a boundary is different from a ‘border’ and from other lines that do not separate territories placed under State sovereignty. It is true that the term ‘border’ in the English language is often used as a synonym to boundary. But it also refers to the mandatory crossing point for travellers leaving or entering a given State and the location of customs services. A border may be established in a different location from the real boundary, as is often the case with borders fixed at airports or railway networks (see eg as to the establishment, in connection with the Eurotunnel railway [Channel Tunnel], of French frontier controls within British territory and British frontier controls within French territory: Art. 5 Sangatte Protocol [1991]).

6 Second, boundaries are at times referred to as ‘frontiers’; but that term may also refer to a frontier zone or a boundary zone, possibly a disputed one, rather than to an established line (Jennings and Watts [eds] Oppenheim’s International Law [1992] 662). In French terminology, ‘frontière’ is used for both, but sometimes the precise meaning is specified by the use of the terms ‘frontière-zone’ and ‘frontière-ligne’ (Blumann [1980] 3, 8). In the Lac Lanoux Arbitration between Spain and France (1957), Spain relied on the concept of ‘boundary zone’, arguing that

the Pyrenean boundary constitutes a zone organized in conformity with a special law, customary in nature, incorporated in international law by the Boundary Treaties which have recognized it, rather than being a limitation on the sovereign rights of bordering States (para. 12).

In the view of the tribunal, however, recourse to the notion of ‘boundary zone’ could not, ‘by the use of a doctrinal vocabulary, add an obligation to those sanctioned by positive law’ (ibid). This entry will, for the sake of precision, use only the term ‘boundary’ to refer to the line separating territories placed under the sovereignty of different States.

7 A boundary is also different from armistice lines and lines separating spheres of influence. Armistice lines are provisional lines drawn primarily to separate the armed forces of belligerents in an armed conflict. Armistice lines do not as such separate territories placed under the sovereignty of two States. They are therefore without prejudice to the question of the sovereignty over the territories which they separate (see eg the armistice ‘Green Line’ in Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory] [Advisory Opinion] [2004] para. 72; Israeli Wall Advisory Opinion [Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory]). In the Commentary to its Resolution 2625, the United Nations General Assembly clarified that, similarly to what is the case in respect of boundaries, every State ‘has the duty to refrain from the threat or use of force to violate international lines of demarcation, such as armistice lines, established by or pursuant to an international agreement to which it is a party or which it is otherwise bound to respect’ (at 122; Friendly Relations Declaration [1970]). To account for the distinction between boundaries and such lines, the General Assembly included the caveat that ‘[n]othing in the foregoing shall be construed as prejudicing the positions of the parties concerned with regard to the status and effects of such lines under their special régimes or as affecting their temporary character’ (ibid). Unlike boundaries, lines separating spheres of influence during colonial periods could separate territories that were not placed under the sovereignty of the States concerned (Colonialism). An example can be found in the Convention between Great Britain and Russia relating to Persia, Afghanistan, and Tibet of 1907, which separated Persia into a British and a Russian sphere of influence, separated not by a boundary but by a ‘ligne’ (‘line’) (Arts I–II). In the African context, colonial powers purported to divide by such lines ‘large tracts of [territory] into mutually recognized spheres or zones of influence’ (Territorial Dispute [Libyan Arab Jamahiriya/Chad] [1994] para. 25; Territorial Dispute Case [Libyan Arab Jamahiriya/Chad]). The Convention between Great Britain and France for the delimitation of their respective possessions to the West of the Niger and of their respective possessions and spheres of influence to the East of that River dated 14 June 1898 illustrates this type of agreement (Hertslet vol 2 [1909] No 241 at 789).

8 Legal terminology has, despite adopting the definition referred to above, sanctioned the term ‘maritime boundaries’ to refer to the limits separating the exclusive economic zone (‘EEZ’) and the continental shelf of opposing or adjacent States. In the Maritime Delimitation in the Black Sea (Romania v Ukraine) (2009), the ICJ, before using this terminology, clarified that

a maritime boundary delimiting the continental shelf and exclusive economic zones is not to be assimilated to a State boundary separating territories of States. The former defines the limits of maritime zones where under international law coastal States have certain sovereign rights for defined purposes. The latter defines the territorial limits of State sovereignty. Consequently, the Court considers that no confusion as to the nature of the maritime boundary delimiting the exclusive economic zone and the continental shelf arises and will thus employ this term (para. 217).

Indeed, ‘neither the exclusive economic zone nor the continental shelf are zones of sovereignty’ (Delimitation of the Maritime Boundary between Guinea and Guinea-Bissau [1985] para. 124); coastal States possess only sovereign rights for the purpose of exploring these maritime zones and exploiting their natural resources (see Arts 74 and 83 United Nations Convention on the Law of the Sea [1982]; ‘UNCLOS’). This entry deals only with land boundaries; it will not therefore discuss maritime boundaries or maritime delimitation.

C. The Boundary-making Process in International Law

9 To establish a boundary, or boundaries, between neighbouring States is ‘to draw the exact line or lines where the extension in space of the sovereign powers and rights’ of one State meets those of another (Aegean Sea Continental Shelf [Greece v Turkey] [1978] para. 85). A classic example is Art. II Treaty of Peace between the Arab Republic of Egypt and the State of Israel (1979) which provides that: ‘The permanent boundary between Egypt and Israel is the recognized international boundary between Egypt and the former mandated territory of Palestine’. A more recent example is Art. 1 Supplementary Agreement between the Kingdom of Saudi Arabia and the State of Kuwait (2019) relating to the so-called Neutral Zone, a desert area that had until then remained undefined:

The line dividing the Neutral Zone, adopted by means of the Supplementary Agreement, shall be considered as part of the international boundary line between the territory of the Kingdom of Saudi Arabia and the territory of the State of Kuwait, with the Kingdom of Saudi Arabia exercising full sovereignty over its territory to the south of that line and the State of Kuwait exercising full sovereignty over its territory to the north of that line.

10 Boundary-making is a multi-stage process. It requires an admixture of legal and technical skills at each and every stage of the process. One classic work distinguished between ‘territorial allocation’, ‘delimitation’, ‘demarcation’, ‘placing of monuments’, and ‘boundary maintenance’, whilst emphasizing the continuity between all these stages:

[T]here is a strong continuity in boundary-making, regardless of gaps or overlaps in the chronological history. In territorial allocation, nowadays, it is seldom possible to ignore the question of boundary site. A treaty of delimitation involves the choice of site and the choice of words with which to define the site. Also it may include provisions for demarcation and administration. Demarcation is not solely an engineering task, for almost inevitably there are fine decisions on site to be made. The treaty cannot define the line as exactly as surveyors can run it. In their fine decisions, the demarcators should consider ease of administration. In the placing of monuments and the cutting of vistas through forests, the demarcators are aiding administration. Their final report may contain suggestions on the future management of border affairs. Finally, among the tasks of boundary administration are physical maintenance, recutting of vistas, replacement of monuments, and demarcations across new roads or bridges or at the scenes of crimes or accidents (Jones [1945] 5).

11 From the perspective of international law, however, the focus is on the distinction between ‘delimitation’ and ‘demarcation’, whereas territorial allocation and delimitation, in common with demarcation with monumentation, are elided. In Land and Maritime Boundary between Cameroon and Nigeria (Cameroon v Nigeria: Equatorial Guinea Intervening) (2002), the Court explained that the ‘delimitation of a boundary consists in its “definition”, whereas the demarcation of a boundary, which presupposes its prior delimitation, consists of operations marking it out on the ground’ (para. 84).

12 There may be good sense in conceiving of ‘territorial allocation’, which is relevant only in the context of diplomatic boundary-making and judicial determination ex aequo et bono, as a part of delimitation. The same is not, however, true of the conflation of demarcation and monumentation. Boundary demarcation includes two separate steps: the first one consists of reporting the boundary line, as agreed at the delimitation stage, on the ground; the second one consists of erecting pillars and other visual aids to materialize the boundary line. Since a territorial dispute may concern the delimitation stage, the demarcation stage, and the monumentation stage (as was the case in the Taba Arbitration, where the parties disagreed on the location of boundary pillars), it is preferable to distinguish the monumentation stage from demarcation, strictly speaking (see Dispute Concerning Certain Boundary Pillars between the Arab Republic of Egypt and the State of Israel [1988]).

13 The distinction between delimitation, demarcation, and monumentation has important legal consequences. Absent special treaty provisions to the contrary, demarcation commissions, and even more so monumentation commissions, are not empowered to change the course of the boundary line as it arises from the delimitation stage. The Eritrea-Ethiopia Boundary Commission thus observed that

a demarcator must demarcate the boundary as it has been laid down in the delimitation instrument, but with a limited margin of appreciation enabling it to take account of any flexibility in the terms of the delimitation itself or of the scale and accuracy of maps used in the delimitation process, and to avoid establishing a boundary which is manifestly impracticable (Delimitation of the Border [Eritrea/Ethiopia]: Observations of the Eritrea–Ethiopia Boundary Commission] [2003] para. 8).

14 Although the Commission noted that ‘there is a practice whereby demarcators may be given some latitude, on various grounds, in demarcating the line which has been delimited by some arbitral or judicial award or by a boundary treaty’ (ibid para. 7), it stressed that this practice was ‘normally based on the agreement of the parties concerned, as expressed in some relevant instrument’ (ibid). The Commission accordingly was not of the view that ‘there is to be derived from that practice a settled rule of customary international law to the effect that demarcators not so expressly empowered nonetheless possess such power’ (ibid).

15 When boundaries are jointly demarcated by the States concerned, observed the tribunal in the Taba Arbitration,

the demarcation is considered as an authentic interpretation of the boundary agreement even if deviations may have occurred or if there are inconsistencies with maps. This has been confirmed in practice and legal doctrine, especially for the case that a long time has elapsed since demarcation (para. 210).

Once the report of the demarcation commission is adopted by the parties, there is a very high bar to be met for a party that subsequently wishes to challenge the boundary line arising from the demarcation.

D. Establishing Boundaries

16 A boundary delimitation between two States possesses always an international character and is ‘obviously a matter of international law’ (Frontier Dispute [Burkina Faso/Mali] [1986] para. 29). It cannot be effected unilaterally by any of the parties involved, or by a third party without their consent. As such, boundary delimitation always requires the consent of the States that have a legal interest in the sovereignty over the territory in question. Parties may in this respect elect to establish their boundaries directly among themselves through boundary agreements (see Section D.1 below) or by delegating the power to establish the boundary to a third entity, that is, an international conference or an international organization (see Section D.2). In the context of the creation of new States, the uti possidetis principle (Uti possidetis Doctrine) serves as a default rule for the establishment of boundaries (see Section D.3).

1. Boundary Agreements

17 When States choose to establish their boundaries through boundary agreements, their determination as to the course of the boundary is not subject to any restriction; it has a constitutive character. Consent is the fundamental principle applicable to the establishment of land boundaries between sovereign States. If the States agree upon the location of the boundary as a matter of international law, the agreed location is the boundary (Arbitration between the Republic of Croatia and the Republic of Slovenia [Final Award] [2017] [‘Croatia/Slovenia Arbitration’] para. 334). Such a delimitation may take into account all the factors that the parties deem relevant. Thus, in Frontier Dispute (Burkina Faso/Mali), the Chamber of the Court observed that: ‘[t]he Parties could at any time have concluded an agreement for the delimitation of their frontier, according to whatever perception they might have had of it’ (para. 46). It has also been argued that there may, in certain circumstances, be an obligation to hold a referendum as regards the question of territorial reapportionment (Peters [2015] 278, 288–90).

18 When States decide to establish themselves the boundaries between them, they may formalize their agreement in a boundary agreement. Such an agreement will tend, in light of its importance and significance for the parties, to be in writing and to qualify as a ‘treaty’ within the meaning of the Vienna Convention on the Law of Treaties (1969) (‘VCLT’) (Treaties). Although nothing prevents States from agreeing orally on a boundary, such an agreement would be unusual. In Belgian Award on the Disputes between the British East Africa Company and the German Witu Company respecting the Farming of Customs and the Administration of the Island of Lamu (1889), which involved an alleged contract that leased the Sultan of Zanzibar’s ports and customs services to the German Witu Company, the tribunal stressed that, ‘if no law prescribes any special form for Conventions between independent States, it is none the less contrary to international usages to contract verbally engagements of that nature and of that importance’ (Hertslet vol 3 [1909] at 894). The same is, in light of their nature and significance, true for boundary agreements. Similarly to all treaties, boundary treaties bind only the States that have concluded them and are, therefore, non-opposable to third States (Bjorge [2021]). A State may accordingly cede only territories over which it holds valid title (Island of Palmas at 870).

19 Here, as in other connections, international law does not ‘attach to matters of form the same degree of importance which they might possess in municipal law’ (Mavrommatis Palestine Concessions [Greece v Great Britain] [Jurisdiction] [1924] 34; Mavrommatis Concessions Cases). Boundary agreements may accordingly be concluded by treaty as well as by tacit agreements. In Sovereignty over Pedra Branca/Pulau Batu Puteh, Middle Rocks and South Ledge [Malaysia/Singapore] [Judgment] [2008] [‘Sovereignty over Pedra Branca’], the Court observed that

[a]ny passing of sovereignty might be by way of agreement between the two States in question…The agreement might instead be tacit and arise from the conduct of the Parties. International law does not, in this matter, impose any particular form. Rather it places its emphasis on the parties’ intentions (para. 120).

In such cases, evidence of the intentions of the parties, not only as to agreement on a boundary line, but also as to the boundary being determined by way of tacit legal agreement, would be decisive (Tacit Agreements).

2. Decisions of International Conferences or International Organizations Establishing Boundaries

20 In some exceptional cases, instead of agreeing on the course of their boundary among themselves, States may refer the determination of the boundary to an international organization or a diplomatic conference. UN Security Council Resolution 687 (1991) is an (imperfect) example of the possibility for an international organization to establish the boundary of States. In that case, the Security Council noted that Iraq and Kuwait had, on 4 October 1963 (Agreed Minutes between the State of Kuwait and the Republic of Iraq regarding the Restoration of Friendly Relations, Recognition and Related Matter [1963]), agreed ‘as independent States’ a boundary agreement, subsequently registered with the United Nations pursuant to Art. 102 UN Charter (Treaties, Registration and Publication). Stressing the ‘need for demarcation of the said boundary’ (UNSC Res 687 at 2), the Security Council called on both parties to ‘respect the inviolability of the international boundary and the allocation of islands’ (ibid at 4), as set out in the 1963 Agreement, and called upon the Secretary-General to lend his assistance in its demarcation. As is apparent, instead of drawing the boundary itself, the Security Council gave force to a boundary that was agreed by the Parties themselves (see Final Report on the Demarcation of the International Boundary between the Republic of Iraq and the State of Kuwait by the United Nations Iraq–Kuwait Boundary Demarcation Commission [21 May 1993]).

21 Boundaries may also be established by a decision of a diplomatic conference. This used to occur relatively often in the context of peace settlements. In Delimitation of the Polish–Czechoslovakian Frontier (Question of Jaworzina) (Advisory Opinion) (1923) the Permanent Court of International Justice (PCIJ) observed that, after the dissolution of the Austro-Hungarian monarchy in the aftermath of the First World War,

[t]he task of ensuring the recognition of the frontiers of the new States and of settling disputes which might arise between them was undertaken by the Principal Allied and Associated Powers represented in the Supreme Council then sitting at Paris. Thus, in the Peace Treaties, side by side with clauses regarding the frontiers of Germany, Austria and Hungary, are to be found clauses by which the Principal Allied and Associated Powers reserve the right subsequently to fix the frontiers of the new States, having obtained in advance the consent of the States enumerated above, to the frontiers thus to be determined (at 20; see also 29).

22 The power of international conferences and organizations to adopt binding decisions as to boundaries of States rests on the conferral of such powers upon them in general or in specific circumstances. Thus, in the Jaworzina advisory opinion, the PCIJ explained that the decision of the Conference of Ambassadors of the Allied Powers dated 28 July 1920, which allocated between Poland and Czechoslovakia the disputed territories of Teschen, Orava, and Spisz, was

the fulfilment at once of a resolution of the Principal Allied Powers and of an agreement between the interested parties. It was taken in accordance with a common desire on the part of all concerned to arrive at a final settlement of the dispute between Poland and Czechoslovakia (Jaworzina at 29).

In the Treaty of Peace with Italy of 10 February 1947, the Allied and Associated Powers, of the one part, and Italy, of the other part, agreed a number of boundaries, or boundary modifications, such as parts of the boundary between Italy and France and between Italy and Yugoslavia.

3. The Creation of States and Boundaries

23 New boundaries are a logical corollary of the creation of States, irrespective of whether the latter occurs as a result of the exercise of the principle of equal rights and self-determination of peoples (see Section D.3(a) below) or through secession (Section D.3(b)).

(a) Boundaries of States Created Through the Exercise of the Right of Peoples to Self-Determination

24 As the ICJ observed in Kosovo,

[d]uring the second half of the twentieth century, the international law of self-determination developed in such a way as to create a right to independence for the peoples of non-self-governing territories and peoples subject to alien subjugation, domination and exploitation (Accordance with International Law of the Unilateral Declaration of Independence in Respect of Kosovo [Advisory Opinion] [2010] [‘Kosovo Advisory Opinion’] para. 79).

In the context of the exercise of the right of self-determination, the identification of the people entitled, in virtue of international law, to exercise its right to independence entails a more or less precise identification of the boundaries of its territories. The boundaries of the newly independent State may, in such cases, be constituted either by the administrative delimitations that divided the possessions of a single colonial power or by the boundaries that delimited possessions belonging to different colonial powers or a colonial power and a local political entity. This is the essence of the uti possidetis iuris principle.

25 In the first scenario, administrative lines which existed prior to accession to independence become international boundaries at the moment of independence. According to the Chamber of the ICJ in Frontier Dispute (Burkina Faso/Mali),

[u]ti possidetis, as a principle which upgraded former administrative delimitations, established during the colonial period, to international frontiers, is therefore a principle of a general kind which is logically connected with this form of decolonization wherever it occurs (para. 23).

Not all administrative boundaries, however, became international boundaries upon independence. In some cases, such as the case of the Northern Cameroons, the General Assembly rejected certain administrative lines between colonies in favour of other lines (Northern Cameroons [Cameroon v United Kingdom] [Preliminary Objections] [1963] at 21–26). In other instances, the attempts of colonial powers to dismember and keep within their possession parts of the colonial territory were also deemed null and void by the General Assembly. As the ICJ observed in Chagos,

peoples of non-self-governing territories are entitled to exercise their right to self-determination in relation to their territory as a whole, the integrity of which must be respected by the administering Power. It follows that any detachment by the administering Power of part of a non-self-governing territory, unless based on the freely expressed and genuine will of the people of the territory concerned, is contrary to the right to self-determination (Legal Consequences of the Separation of the Chagos Archipelago from Mauritius in 1965 [Advisory Opinion] [2019] para. 160).

From this practice it appears that administrative delimitations under domestic law become boundaries at the moment of accession of independence, because of their correspondence with the territorial scope of the territory of the people entitled to exercise the right to independence through self-determination. It is not their character as administrative delimitations per se that is decisive. No conflict may therefore exist between the right of self-determination and the principle of uti possidetis iuris. The right of self-determination determines the scope of the territory over which a people is entitled to exercise its right to independence: uti possidetis iuris serves to determine the boundaries of this territory.

26 In the second and third scenarios relating to the creation of States through self-determination, the territory of the newly independent State is delimited either through agreements between colonial powers, as was the case with Germany and the United Kingdom in Land and Maritime Boundary between Cameroon and Nigeria, or through agreements between an existing State and a colonial power, as was the case between Libya and France in Territorial Dispute (Libyan Arab Jamahiriya/Chad). In Burkina Faso/Mali the Chamber of the Court observed that ‘[t]here is no doubt that the obligation to respect pre-existing international frontiers in the event of a State succession derives from a general rule of international law, whether or not the rule is expressed in the formula uti possidetis’ (para. 24). Article 11 (a) 1978 Vienna Convention on Succession of States in respect of Treaties reflects this general rule of international law when it declares that ‘[a] succession of States does not as such affect: (a) a boundary established by a treaty’. Sub-paragraph (b) of the same provision provides that State succession does not affect ‘obligations and rights established by a treaty and relating to the regime of a boundary’ (State Succession in Treaties).

(b) Boundaries Resulting from the Creation of States in the Context of Secession

27 The ICJ observed in the Kosovo Advisory Opinion that a great many States came into independence as a result of the exercise of the principle of equal rights and self-determination of peoples. It noted that ‘[t]here were, however, also instances of declarations of independence outside this context’ (para. 79). Declarations of independence outside the context of the exercise of the right to independence based on the principle of self-determination occur in three circumstances. First, declarations of independence may be authorized by an agreement between the existing State and a secessionist movement. This was the case with the Comprehensive Peace Agreement between the Government of the Republic of the Sudan and the Sudan People’s Liberation Movement/Sudan People’s Liberation Army of 9 January 2005. Secondly, declarations of independence may be authorized by the relevant domestic law. Certain States, such as Ethiopia (Art. 39 1995 Constitution of the Federal Democratic Republic of Ethiopia), Saint-Kitts and Nevis (Art. 113 1983 Saint-Kitts and Nevis Constitution), Uzbekistan (Art. 74 Constitution of Uzbekistan), and the United Kingdom (Agreement between the United Kingdom Government and the Scottish Government on a Referendum on Independence for Scotland [15 October 2012]), acknowledge within their domestic legal orders a right to independence to their domestic constituencies or some of them. Thirdly, declarations of independence may also be made without being based on any legal authorization, whether domestic or international.

28 When a State is created based on an agreement between a parent State and a secessionist entity, the agreement specifies the territorial scope of the secessionist entity (see for instance, Arts 1.1 and 5 Protocol between the Government of the Sudan and the Sudan People’s Liberation Movement/Army on the Resolution of the Abyei Conflict [2004] and the discussion of the Abyei Formula in the Abyei Arbitration [2009] paras 571–672; Abyei Arbitration). The same goes for declarations of independence based on a domestic law. When the creation of a State by a secessionist entity does not have any legal basis, the territorial scope of secessionist entities is not determined at the moment when the secessionist entity breaks away from the parent State. The ‘territory’ of the secessionist entity would in such a scenario arise from the control over a territory and the acceptance, more or less forced, of this fact by the existing State. The boundary would therefore follow the ensuing consensus, if any, between the two parties.

29 State practice, especially concerning the accessions to independence of States in the Socialist Federal Republic of Yugoslavia and the USSR, shows that new States adopted as international boundaries the pre-existing administrative lines. It is in this sense that it can be said that uti possidetis is ‘a general principle, which is logically connected with the phenomenon of the obtaining of independence, wherever it occurs’ (Frontier Dispute [Burkina Faso/Mali] para. 20; Croatia/Slovenia Arbitration para. 336). Nevertheless, a boundary arising from an instance of secession which is authorized neither under domestic nor international law may be fixed in accordance with other criteria agreed upon by the parties concerned.

4. Maps

30 A boundary line may be drawn with the assistance of the most variegated objects: geodetical points, church towers, railway tracks, ditches, bridges, canals, roads, river banks, fountainheads, locks, hills or mountain tops, farms, cemeteries, swamps, oases, trees, or monuments. Other boundary lines are drawn summarily: they follow, for example, the course of one of the banks or the thalweg of a river, are indicated by a meridian or parallel of latitude, or by straight lines (Verzijl [1970] 522). Irrespective of the elements or criteria used to delimit boundaries, the boundary line tends to be materialized on a map.

31 States often annex maps to their boundary agreements. Maps may also be annexed to a domestic statute which establishes an administrative line, which by virtue of the uti possidetis principle subsequently becomes a boundary at the moment of independence. States also draw up maps that are not annexed to any legal instrument. The legal value of maps differs in these different scenarios. When maps are annexed to treaties, they form part of the context of the treaty pursuant to Art. 31 (2) VCLT; account may therefore need to be taken of them in the interpretation of the treaty. Maps represent in such cases ‘a physical expression of the will’ of the States involved (Frontier Dispute [Burkina Faso/Mali] para. 56). Conversely, when maps are not annexed to a legal instrument that establishes a boundary, international courts and tribunals have treated them with ‘a considerable degree of caution’ (ibid). In such a case, ‘[t]he actual weight to be attributed to maps as evidence depends on a range of considerations. Some of these relate to the technical reliability of the maps’ (ibid para. 55; see also Island of Palmas at 852, which holds that ‘only with the greatest caution can account be taken of maps in deciding a question of sovereignty’). In any case, such maps

cannot in themselves alone be treated as evidence of a frontier, since in that event they would form an irrebuttable presumption, tantamount in fact to legal title. The only value they possess is as evidence of an auxiliary or confirmatory kind, and this also means that they cannot be given the character of a rebuttable or juris tantum presumption such as to effect a reversal of the onus of proof (Frontier Dispute [Burkina Faso/Mali] para. 56).

E. The Judicial Settlement of Boundary Disputes

32 When States are involved in boundary disputes, that is, situations where the claim of one party with respect to the course of the boundary line is ‘positively opposed by the other’ (South West Africa Cases [Ethiopia v South Africa; Liberia v South Africa] [Preliminary Objections] [1962] at 328; South West Africa/Namibia [Advisory Opinions and Judgments]), they may decide to settle their dispute either through direct and friendly, that is, diplomatic, means or alternatively arbitral or judicial means. It is the logic of international law, here no less than in other contexts, that ‘[r]ecourse to delimitation by arbitral or judicial means is in the final analysis simply an alternative to direct and friendly settlement between the parties’ (Delimitation of the Maritime Boundary in the Gulf of Maine Area [Canada/United States of America] [1984] para. 22).

33 Since the jurisdiction of international courts and tribunals is based upon consent, arbitral or judicial settlement of boundary disputes requires States to consent to the jurisdiction of the tribunal or court in question (International Courts and Tribunals, Jurisdiction and Admissibility of Inter-State Applications). States have often attempted to have international tribunals decline to exercise their jurisdiction in boundary disputes because of the interests of a third party in the location of the tripoint where the boundary between the disputing parties meets that of a third party. In Frontier Dispute (Burkina Faso/Mali), the Chamber of the ICJ noted that ‘its jurisdiction is not restricted simply because the end-point of the frontier lies on the frontier of a third State not a party to the proceedings’ (para. 46). The Court observed in Land and Maritime Boundary (Cameroon v Nigeria), addressing Nigeria’s contention that Chad was an indispensable third party in the boundary dispute between Cameroon and Nigeria, that:

Any redefinition of the point where the frontier between Cameroon and Nigeria meets the Chad-Cameroon frontier could in the circumstances only lead to a moving of the tripoint along the line of the frontier in the Lake between Chad and Cameroon. Thus, the legal interests of Chad as a third State not party to the case do not constitute the very subject-matter of the judgment to be rendered on the merits of Cameroon’s Application; and therefore, the absence of Chad does not prevent the Court from proceeding to a specification of the border between Cameroon and Nigeria in the Lake ([Preliminary Objections] [1998] para. 79).

34 Judicial decisions establishing a boundary have, except when rendered ex aequo et bono, a declaratory character. In such a case, the tribunal is bound to determine the course of the boundary line only on the basis of international law, a mandate which entails significant restrictions on the exercise of its jurisdiction (see Section E.1 below). When a tribunal determines the course of a boundary, consequently, ‘a clarification is made of a given legal situation with declaratory effect from the date of the legal title upheld by the court’ (Frontier Dispute [Burkina Faso/Mali] para. 17). The construction by international courts and tribunals of the relations between the titles invoked by the parties and the effectivités submitted by them may also have a significant impact on the outcome (see Section E.2).

1. The Impact of the Mandate of International Courts and Tribunals on the Settlement of Boundary Disputes Based on International Law

35 When it is seised of a boundary dispute, an international court or tribunal is bound by the choice of the parties concerning the law applicable to the dispute (Applicable Law). States will in the large majority of cases request from the tribunal a decision as to their disputes in accordance with ‘international law’.

36 International courts and tribunals are rarely (if ever) authorized to settle boundary disputes ex aequo et bono. A rare example where a tribunal construed its mandate as including this power concerned the Dispute over the Inter-Entity Boundary in Brcko Area (Republika Srpska v Federation of Bosnia and Herzegovina) (1997) 36 ILM 409–10 para. 38 and 421 para. 75. The relevant compromis stipulated that ‘the Arbitrators shall apply relevant legal and equitable principles’ (see Annex 2 Art. V (3) General Framework Agreement for Peace in Bosnia and Herzegovina [1995] reproduced ibid at 400, para. 3). In that case, instead of drawing a boundary line that would attribute the disputed territory to one or the other of the parties, as each party claimed, the arbitral tribunal established ‘an interim supervisory regime in the Brcko area’ (ibid 433–35, para. 104; [Supplemental Award] [1998] para. 27). The arbitral tribunal subsequently delivered a decision placing the disputed territory under a condominium régime that would be governed by a new institution to be known as ‘The Brcko District of Bosnia and Herzegovina’ (Arbitral Tribunal for Dispute over Inter-Entity Boundary in Brcko Area [Federation of Bosnia and Herzegovina v Republika Srpska] [Final Award] [1999] paras 9–12; Condominium and Coimperium).

37 When the mandate of international tribunals seised of boundary disputes is to settle the dispute in accordance with international law, this limited mandate calls tribunals to focus only on legal considerations and to exclude any other considerations. Thus the tribunal in the Croatia/Slovenia Arbitration, which was tasked with the determination of the boundary between the two States in accordance with international law, remarked that:

The Tribunal has neither the right nor the legal power to decide upon the course of the boundary except by applying the rules and principles of international law. Factors that are legally irrelevant, or which the Parties have expressly decided should be excluded from consideration, must not be taken into account by the Tribunal in reaching its decision. The Tribunal is required to decide the matter from the legal, and not from the historical or political or sociological perspective (para. 335).

38 This is in keeping with the observation of the ICJ in Temple of Preah Vihear (Cambodia v Thailand) (Merits) (1962) that ‘[t]he Parties have also relied on other arguments of a physical, historical, religious and archaeological character, but the Court is unable to regard them as legally decisive’ (at 15 [emphasis added]). This limit has significant consequences for international courts and tribunals. The most important of those consequences specific to boundary disputes, that is, those relating to the court or tribunal’s ability to take into account human considerations (see Section E.1(a) below), equitable considerations (Section E.1(b)) and, finally, the doctrine of natural boundaries (Section E.1(c)) will be considered in turn.

(a) Impact of Human Considerations

39 When the mandate of an international court or tribunal is to determine a boundary line in accordance with international law, it cannot in the ordinary course of events take into account the impact of the boundary line on the populations living in the disputed area and their activities. For instance, in Land, Island and Maritime Frontier Dispute (El Salvador/Honduras: Nicaragua Intervening) (1992), the Chamber of the Court noted that

[t]he effect of the Chamber’s Judgment will … not be that certain areas will “become” part of Honduras; the Chamber’s task is to declare what areas are, and what are not, already part of the one State and the other. If Salvadorians have settled in areas of Honduras, neither that fact, nor the consequences of the application of Honduran law to their properties, can affect the matter (para. 97 [emphasis in the original]).

In such cases, international courts or tribunals invite the parties to engage in negotiations to address the inconveniences of the boundary delimitation on the population living in the boundary area (see eg ibid para. 66; also Land and Maritime Boundary between Cameroon and Nigeria [2002] para. 317). It should, however, be stressed that the decision on the course of the boundary does not affect property rights acquired in good faith (Frontier Dispute [Benin/Niger] para. 124; Abyei Arbitration para. 755 fn 1253). In the same vein, parties must comply with their human rights obligations.

40 The arbitral tribunal in the Abyei Arbitration case seems to have crafted a limited exception to the restricted role that equity contra legem may play in boundary disputes. It held that that pre-existing traditional rights might result in spatial adjustments in the delimiting of boundaries, especially if a boundary delimitation would entail ‘catastrophic repercussions for the livelihood and economic well-being of the population of the countries concerned’ (para. 754 fn 1253). It is arguable that this is no more than the expression, in the specific context of boundary delimitation, of an aspect of the general principle of international law, codified in common Art. 1 (2) International Covenant on Civil and Political Rights (1966) and the International Covenant on Economic, Social and Cultural Rights (1966), that ‘[i]n no case may a people be deprived of its own means of subsistence’.

(b) Impact of Equitable Considerations

41 The mandate that States typically give international courts and tribunals, to decide their boundary disputes in accordance with international law, disables the court or tribunal from deciding these disputes ex aequo et bono, except where there is specific authorization to this effect from the parties. In Frontier Dispute (Burkina Faso/Mali), the Chamber stressed that it could not decide the matter ex aequo et bono; as ‘the Parties have not entrusted it with the task of carrying out an adjustment of their respective interests, it must also dismiss any possibility of resorting to equity contra legem’ (para. 28). Referring specifically to the African context, the Chamber stressed that

to resort to the concept of equity in order to modify an established frontier would be quite unjustified. Especially in the African context, the obvious deficiencies of many frontiers inherited from colonization, from the ethnic, geographical or administrative standpoint, cannot support an assertion that the modification of these frontiers is necessary or justifiable on the ground of considerations of equity. These frontiers, however unsatisfactory they may be, possess the authority of the uti possidetis and are thus fully in conformity with contemporary international law (ibid para. 149).

42 Articles 74 (1) and 83 (1) UNCLOS mandate an ‘equitable solution’ as the result of the delimitation process of the EEZ and the continental shelf. The position is different as regards the judicial settlement of land boundary disputes. Although equity has a certain role to play in terrestrial boundary disputes, this is much more modest than the role it plays in maritime delimitation (Weil [1989] 1024).

43 It often happens that the material submitted by the Parties does not allow the Court to identify the exact course of the boundary in the disputed area. In such cases, tribunals still have to discharge their mandate and decide the dispute that is submitted to them. For that purpose, they may, and often do in practice, take into account equitable considerations in shaping their decision as to the exact course of the boundary. For instance, in Indo–Pakistan Western Boundary (Rann of Kutch) between India and Pakistan (1968) the tribunal decided that the two deep inlets on either side of Nagar Parkar fell on the Pakistani side of the boundary (Rann of Kutch Arbitration [Indo-Pakistan Western Boundary]). The reasoning was that:

it would be inequitable to recognise these inlets as foreign territory. It would be conducive to friction and conflict. The paramount consideration of promoting peace and stability in this region compels the recognition and confirmation that this territory, which is wholly surrounded by Pakistan territory, also be regarded as such ([Opinion of the Chairman] at 571).

44 In the Frontier Dispute (Burkina Faso/Mali), the Chamber of the Court reached the conclusion that the boundary went through the Soum pool. The materials submitted by the Parties, however, did not allow the Chamber to determine how the boundary divided the Soum pool between them. The Chamber of the Court thus decided that, ‘in the absence of any precise indication in the texts of the position of the frontier line, the line should divide the pool of Soum in two, in an equitable manner’ (para. 150). In Frontier Dispute (Burkina Faso/Niger) (2013), the Court made similar findings. It noted that the boundary between the two parties reached the Sirba River in the region of Bossébangou. There was, however, no evidence that the river had in this area been attributed entirely to one or the other party. The Court accordingly held that ‘the requirement concerning access to water resources of all the people living in the riparian villages is better met by a frontier situated in the river than on one bank or the other’ (para. 101). Subsequently the Court clarified that the boundary in the river ran through the median line of the river (ibid paras 101–2). As is apparent from these examples, international courts and tribunals apply equity infra legem, that is, ‘that form of equity which constitutes a method of interpretation of the law in force, and is one of its attributes’ (Frontier Dispute [Burkina Faso/Mali] para. 28).

(c) Impact on the Doctrine of Natural Boundaries

45 International law does not subscribe to the doctrine of ‘natural boundaries’, in the sense that it is of course not the case that, where a natural boundary can be discerned, the political delimitation necessarily follows the line suggested by it.

46 This is so although the doctrine was supported by the early scholars of international law (see eg Grotius De Jure Belli ac Pacis Book II [orig 1646] FW Kelsey (tr) [Clarendon Press Oxford 1925]) and may have played a role in early arbitrations (see eg The Guiana Boundary Case [Brazil v Great Britain] [1904] [which refers to ‘the lines traced by nature’ at 22] and the Border Dispute between Honduras and Nicaragua [1906] [which refers to the River Coco as ‘the most precise and natural boundary which could be desired’ at 115]). In fact, the concept of natural boundaries has always been highly political. Such ‘natural’ frontiers constitute something that is dearly wanted, such as the sustained struggle of France to reach the ‘natural’ Rhine boundary (temporarily achieved by Art. VI Treaty of Peace between the Emperor, the Empire and France, signed at Lunéville [1801], supplemented by Art. VI Treaty between France and Holland for the Prohibition of Commerce with England [1810]) (Verzijl [1970] 516). As one commentator observed of France’s boundary difficulties, ‘[t]he odd idea arose that the Rhine was a natural border, whereas the vocation of any river is to be navigated and bridged’ (de Gramont [1970] at 6). As a general proposition, there is much truth in the observation that natural boundaries are in fact highly political in that they are ‘advocated only when they involve an accession of territory’ (Taylor [1948] 124).

47 In fact boundaries are always man-made: a natural feature becomes part of a boundary only when the parties so decide. In the Gulf of Maine case, the Chamber of the Court observed that ‘a delimitation, whether of a maritime boundary or of a land boundary, is a legal-political operation, and that it is not the case that where a natural boundary is discernible, the political delimitation necessarily has to follow the same line’ (para. 56). As a result, in boundary disputes involving a natural boundary, it is important to determine first what is the course of the boundary line according to the applicable legal instrument. In Certain Activities Carried Out by Nicaragua in the Border Area (Costa Rica v Nicaragua) and Construction of a Road in Costa Rica along the San Juan River (Nicaragua v Costa Rica) (2015) (‘Certain Activities’), the Court had to determine whether the caño dredged by Nicaragua formed part of the boundary between Nicaragua and Costa which, according to the 1858 Treaty of Limits, runs along the Costa Rican bank of the San Juan river. For this purpose, the Court reviewed first maps, affidavits, aerial and satellite images of the caño before concluding that it could not form part of the boundary since it was not a ‘navigable outlet of commerce’ as required by the 1858 Treaty of Limits (paras 65–92). This somewhat unconventional approach did not lead to any consequences since, in the final analysis, the terms of the 1858 Treaty of Limits prevailed (Certain Activities Carried Out by Nicaragua in the Border Area [Costa Rica v Nicaragua] and Construction of a Road in Costa Rica along the San Juan River [Nicaragua v Costa Rica]).

48 Whereas the parties remain free in the context of diplomatic boundary-making to choose natural features as boundaries, international courts and tribunals, whose decisions are based on law, do not enjoy such freedom. There are examples, however, of situations where boundary commissions are specifically instructed to determine ‘les frontières des deux Etats, en tenant compte de la configuration des montagnes’ (‘the boundaries of the two States by taking into account the configuration of the mountains’ [translation by the authors]) (Art. 3 Treaty of Turin [1860] at 34) or directed ‘to adjust the frontier to geographical…conditions’ (Art. 5 (5) Treaty of Peace with Italy; see Bardonnet [1981] 35, 49–51).

49 Even where this is not the case, natural features may play a role in the determination. This could be based on the presumption that the parties desired ‘natural and visible frontiers’ (Temple of Preah Vihear at 35). It could alternatively be based on inferences drawn from the practice of the parties with respect to agreed sectors of the boundary, as was the case in Land, Island and Maritime Frontier Dispute (El Salvador/Honduras: Nicaragua Intervening). In that judgment, the Chamber of the Court observed the predominance of local features, particularly rivers, in the sectors of the boundary upon which the parties agreed, on which basis it felt ‘entitled and bound to have an eye to the topography of each land sector’ (para. 46). It explained its approach as follows:

When … the very many instruments cited, even after minute examination, are found to give no clear and unambiguous indication, the Chamber has felt it right similarly to take some account of the suitability of certain topographical features to provide an identifiable and convenient boundary. The Chamber is here appealing not so much to any concept of “natural frontiers”, but rather to a presumption underlying the boundaries on which the uti possidetis juris operates. Considerations of this kind have been a factor in boundary-making everywhere, and accordingly are likely, in cases otherwise dubious, to have been a factor also with those who made the provincial boundaries previous to 1821 (ibid).

50 International courts and tribunals have similarly developed a practice in dealing with river boundaries when the relevant instrument does not allow the determination of the exact course of the boundary in the river. In Kasikili/Sedudu Island (Botswana/Namibia) (1999), the Court observed that

[t]reaties or conventions which define boundaries in watercourses nowadays usually refer to the thalweg as the boundary when the watercourse is navigable and to the median line between the two banks when it is not, although it cannot be said that practice has been fully consistent (para. 24).

Later, in Frontier Dispute (Benin/Niger), the Chamber of the Court considered that

in view of the circumstances, including the fact that the river is not navigable, a boundary following the median line of the Mekrou would more satisfactorily meet the requirement of legal security inherent in the determination of an international boundary (para. 144).

51 The Eritrea–Ethiopia Boundary Commission (EEBC) faced a similar difficulty: the materials submitted by the parties did not allow a clear identification of the course of the boundary in the watercourses in question. Instead of deciding the boundary line using equity infra legem, as in the cases mentioned above, the tribunal directed the parties to address this issue during the hearings. As the Boundary Commission observed,

[b]oth favoured the adoption in principle of the main channel as the line of division. Neither referred to the line of the deepest channel. Neither favoured the fixing of a permanent line in rivers determined by reference to coordinates. Both favoured the deferment to the demarcation stage of the decision regarding the line within rivers and considered that the Parties should be consulted further on the matter at that stage, bearing in mind, amongst other factors, that different considerations might apply to different parts of the rivers (Decision regarding Delimitation of the Border between Eritrea and Ethiopia [2002] para. 7.2).

52 The Commission decided to proceed accordingly (ibid para. 7.3). At the demarcation stage, the Commission determined, after ‘[h]aving considered the submissions and comments of the Parties’, that ‘the boundary is the middle of the main channel (the channel of greatest volume) and will move in accordance with any change in position of the middle of the main channel’ (Delimitation of the Border [Eritrea–Ethiopia]: Demarcation Instructions [2003] para. 20 (b)).

2. The Relation Between Titles and Effectivités

53 The ICJ made a landmark clarification in 1986 of the law applicable to the settlement of boundary disputes by holding that titles prevail over contradicting effectivités (see Section E.2(a) below). International courts and tribunals have nevertheless held that, in certain circumstances, the silence of the holder of the title confronted with effectivités performed in its territory could amount to acquiescence (Section E.2(b)).

(a) The Primacy of Titles over Effectivités

54 Reuter observed that ‘[a]ll territorial disputes, without exception, lead the judge to compare and assess titles, on the one hand, and facts of effective occupation, facts of effective exercise of sovereignty, on the other hand’ (Temple of Preah Vihear [Cambodia v Thailand] ICJ Pleadings 1962 at 545 [Reuter]). Title will in boundary disputes rest either on a boundary agreement or on the uti possidetis iuris principle. As for effectivités, they encompass all ‘manifestations of the display of territorial sovereignty’ (Sovereignty over Pedra Branca para. 121) or, in the context of uti possidetis, ‘conduct of the administrative authorities as proof of the effective exercise of territorial jurisdiction in the region during the colonial period’ (Frontier Dispute [Burkina Faso/Mali] para. 63).

55 Prior to the Burkina Faso/Mali judgment, scholars had argued that the judicial settlement of boundary disputes and territorial disputes were subject to different rules. For this purpose, they defined boundary disputes as contradictory claims with respect to the course of the boundary line and territorial disputes as contradictory claims to the sovereignty over a discrete piece of territory. Effectivités would for those scholars play a significant role in territorial disputes, whereas title would be paramount in boundary disputes. In Frontier Dispute (Burkina Faso/Mali), the Chamber of the Court challenged this doctrinal construction. It emphasized that

in the great majority of cases, including this one, the distinction outlined above is not so much a difference in kind but rather a difference of degree as to the way the operation in question is carried out. The effect of any delimitation, no matter how small the disputed area crossed by the line, is an apportionment of the areas of land lying on either side of the line (para. 17).

The Chamber felt obliged to dispel a misunderstanding concerning the concept of ‘title’ which might be at the root of this confusion, stressing that ‘the concept of title may also, and more generally, comprehend both any evidence which may establish the existence of a right, and the actual source of that right’ (ibid para. 18). Both territorial disputes and boundary disputes are accordingly settled on the basis of the same legal principles (Territory, Acquisition), a cornerstone principle of which is the primacy of title over effectivités.

56 In Frontier Dispute (Burkina Faso/Mali), the Chamber of the Court clarified the relations between titles and effectivités in the context of uti possidetis, and it did so stressing the need to distinguish between different scenarios:

Where the act corresponds exactly to law, where effective administration is additional to the uti possidetis juris, the only role of effectivité is to confirm the exercise of the right derived from a legal title. Where the act does not correspond to the law, where the territory which is the subject of the dispute is effectively administered by a State other than the one possessing the legal title, preference should be given to the holder of the title. In the event that the effectivité does not co-exist with any legal title, it must invariably be taken into consideration. Finally, there are cases where the legal title is not capable of showing exactly the territorial expanse to which it relates. The effectivités can then play an essential role in showing how the title is interpreted in practice (para. 63).

57 The Court has in its subsequent jurisprudence applied the Burkina Faso/Mali holding in other cases, irrespective of whether the title concerned was uti possidetis or a boundary treaty (Land, Island and Maritime Frontier Dispute [El Salvador/Honduras: Nicaragua Intervening] paras 61–62; Sovereignty over Pulau Ligitan and Pulau Sipadan [Indonesia/Malaysia] [2002] para. 126; Land and Maritime Boundary between Cameroon and Nigeria [2002] para. 68; Frontier Dispute [Benin/Niger] para. 47; and Territorial and Maritime Dispute between Nicaragua and Honduras in the Caribbean Sea [Nicaragua v Honduras] [2007] paras 151–58). The holding that title prevailed over effectivités has also been applied by arbitral tribunals (Croatia/Slovenia Arbitration para. 262). The rationale for the primacy of title over effectivités may be found in the Latin maxim ex iniura ius non oritur. As the Court noted in Cameroon v Nigeria, effectivités performed in violation of a boundary line are to be characterized legally as ‘contra legem’, that is, in breach of the law (in this case the territorial sovereignty of the other State). They cannot as such produce any legal effect, including changing the course of the boundary line (Land and Maritime Boundary between Cameroon and Nigeria [Cameroon v Nigeria: Equatorial Guinea intervening] [2002] para. 64).

(b) Acquiescence

58 International courts and tribunals have relied on the lack of formalism of international law to accept that, in certain circumstances, they can determine that title may have changed hands through acquiescence. Summarizing the case-law on the matter, the ICJ has explained that:

Under certain circumstances, sovereignty over territory might pass as a result of the failure of the State which has sovereignty to respond to conduct à titre de souverain of the other State or, as Judge Huber put it in the Island of Palmas case, to concrete manifestations of the display of territorial sovereignty by the other State (Island of Palmas Case (Netherlands/United States of America), Award of 4 April 1928, RIAA, Vol. II, (1949) p. 839). Such manifestations of the display of sovereignty may call for a response if they are not to be opposable to the State in question. The absence of reaction may well amount to acquiescence…That is to say, silence may also speak, but only if the conduct of the other State calls for a response (Sovereignty over Pedra Branca para. 121).

59 As is apparent from Pedra Branca, it is the conduct of both parties that is relevant: the Court found that there had been ‘a convergent evolution of the positions of the Parties regarding title’ to the territories in question (Sovereignty over Pedra Branca para. 276).

60 The Court has so far relied upon acquiescence as a title of territorial sovereignty only in two cases, involving both the relations between colonial powers and local political entities: the Sultanate of Johor and Great Britain in Sovereignty over Pedra Branca and the Kingdom of Siam and France in Temple of Preah Vihear. In the latter case, the Court inferred acquiescence from the silence of the Kingdom of Siam to the publication of map which showed a boundary course different from the one agreed by the two parties in the 1904 Treaty. In light of the circumstances of the case, Siam was held to have acquiesced to the boundary line as described in the map (Temple of Preah Vihear at 23).

61 Title based on acquiescence tends to give rise to controversies since it rests, in the final analysis, on the Court’s interpretation of the significance of the conduct of the parties. More generally, a finding of acquiescence cannot be easily presumed as such an approach might subvert the primacy of titles over effectivités. Acquiescence should therefore be established by evidence that the holder of territorial sovereignty wanted to transfer sovereignty over its territory to the State which was performing effectivités and that it wished to do so through its silence (Hébié [2023]). In Sovereignty over Pedra Branca the Court highlighted ‘the central importance in international law and relations of State sovereignty over territory and of the stability and certainty of that sovereignty’ (para. 122). It concluded that

any passing of sovereignty over territory on the basis of the conduct of the Parties, as set out above, must be manifested clearly and without any doubt by that conduct and the relevant facts. That is especially so if what may be involved, in the case of one of the Parties, is in effect the abandonment of sovereignty over part of its territory (ibid).

There is every reason to believe that, in the event of a dispute between two parties as to whether their terrestrial boundary has been agreed tacitly, the evidence of such intention would need to be compelling in order for a court or tribunal to recognize such a boundary (mutatis mutandis, Maritime Delimitation in the Indian Ocean [Somalia v Kenya] [2021] para. 52).

F. Fundamental Principles of International Law Applicable to Boundaries

62 Before discussing the fundamental principles of international law applicable to boundaries, it is necessary first to clear some confusion with respect to two principles of international law that are allegedly applicable to African boundaries (see also Boundary Disputes in Africa). The first one is the so-called principle of the respect for boundaries inherited from colonialization. If certain African States, perhaps under the influence of counsel, have referred to such a principle in the context of proceedings before the Court, no such a principle exists in African Union law and practice. Thus, in Resolution 16 ‘Border Disputes among African States’ (1964), Member States of the Organization of the African Unity solemnly pledged ‘themselves to respect the borders existing on their achievement of national independence’ (African Union [AU]). Similarly, Art. 4 (b) Constitutive Act of the African Union (2000) provides that the Union shall function in accordance, inter alia, with the principle of the ‘respect of borders existing on achievement of independence’. The choice of words in these two and related instruments is significant.

63 The second principle allegedly applicable to African boundaries is the so-called principle of the intangibility of boundaries. Burkina Faso and Mali introduced this terminology in their compromis in the Frontier Dispute (Burkina Faso/Mali). The Chamber of the Court elided it with the principle uti possidetis, stressing that it is the application of the principle of uti possidetis iuris that ‘gives rise to this respect for intangibility of frontiers’ (para. 20). The so-called principle of intangibility of boundaries should therefore not be understood literally, as signalling that African boundaries existing upon the accession to independence may not be changed by agreement by the newly independent countries. These States remain, on the contrary, free to adjust their boundaries. As the Court phrased it in Territorial Dispute (Libyan Arab Jamahiriya/Chad),

[t]he fixing of a frontier depends on the will of the sovereign States directly concerned. There is nothing to prevent the parties from deciding by mutual agreement to consider a certain line as a frontier, whatever the previous status of that line. If it was already a territorial boundary, it is confirmed purely and simply. If it was not previously a territorial boundary, the agreement of the parties to ‘recognize’ it as such invests it with a legal force which it had previously lacked (para. 45).

OAU Resolution 16 and Art. 4 (b) Constitutive Act of the AU prohibit are unilateral attempts to change boundaries existing upon achievement of independence. As the Chamber explained in Frontier Dispute (Burkina Faso/Mali), the

obvious purpose [of the adoption of the uti possidetis iuris principle] is to prevent the independence and stability of new States being endangered by fratricidal struggles provoked by the challenging of frontiers following the withdrawal of the administering power (para. 20).

64 Unlike the two so-called principles discussed above, the principle of finality and stability (see Section F.1 below) and the principle of the inviolability of boundaries (Section F.2) are well-established principles of international law.

1. The Principle of the Stability and Finality of Boundaries

65 The principle of the finality and stability of boundaries derives from the very object and purpose of boundary agreements. When two States establish a boundary between them, ‘one of the primary objects is to achieve stability and finality’ (Temple of Preah Vihear at 34). Several consequences attach to this interpretation of boundary treaties, which may be strengthened as relevant by preambular provisions.

66 First, ‘any article designed to fix a frontier should, if possible, be so interpreted that the result of the application of its provisions in their entirety should be the establishment of a precise, complete and definitive frontier’ (Interpretation of Article 3, Paragraph 2, of the Treaty of Lausanne [Frontier between Turkey and Iraq] [Advisory Opinion] [1925] at 20). Thus, in Territorial Dispute (Libyan Arab Jamahiriya/Chad), the Court, interpreting Art. 11 Treaty of Friendship and Good-Neighbourliness between France and Libyan Arab Jamahiriya (1955), which included an indication that it was concluded only for 20 years, emphasized that ‘[t]here is nothing in the 1955 Treaty to indicate that the boundary agreed was to be provisional or temporary; on the contrary it bears all the hallmarks of finality’ (para. 72).

67 Secondly, the establishment of a boundary is a fact which, from the outset, has ‘a legal life of its own’, independently of the fate of the legal instrument that established it. Grounds of nullity that may affect the treaty accordingly leave unaffected the boundary established by it. As the Court observed in the Temple of Preah Vihear, stability and finality of boundaries would prove

impossible if the line so established can, at any moment, and on the basis of a continuously available process, be called in question, and its rectification claimed, whenever any inaccuracy by reference to a clause in the parent treaty is discovered. Such a process could continue indefinitely, and finality would never be reached so long as possible errors still remained to be discovered. Such a frontier, so far from being stable, would be completely precarious (at 34).

Article 62 (2) (a) VCLT embodies the same logic: it provides that a State may not invoke the plea of fundamental change of circumstances to withdraw from or to terminate a treaty ‘if the treaty establishes a boundary’ (Treaties, Fundamental Change of Circumstances). In Territorial Dispute (Libyan Arab Jamahiriya/Chad), the Court stated more generally that:

A boundary established by treaty thus achieves a permanence which the treaty itself does not necessarily enjoy. The treaty can cease to be in force without in any way affecting the continuance of the boundary…This is not to say that two States may not by mutual agreement vary the border between them; such a result can of course be achieved by mutual consent, but when a boundary has been the subject of agreement, the continued existence of that boundary is not dependent upon the continuing life of the treaty under which the boundary is agreed (para. 73).

68 The Court’s observation is an expression of the more general and fundamental principle that ‘a territorial régime established by treaty “achieves a permanence which the treaty itself does not necessarily enjoy” and the continued existence of that régime is not dependent upon the continuing life of the treaty under which the régime is agreed’ (Territorial and Maritime Dispute [Nicaragua v Colombia] [Preliminary Objections] [2007] para. 89; Dispute regarding Navigational and Related Rights [Costa Rica v Nicaragua] [2009] para. 68).

2. The Principle of the Inviolability of Boundaries

69 The principle of the inviolability of international boundaries is a corollary of the obligation of States to respect the sovereignty and territorial integrity of other States (Territorial Integrity and Political Independence). Respect for territorial integrity is a cornerstone principle of contemporary international law. As a result, boundaries are protected by Art. 2 (4) UN Charter, which provides that ‘[a]ll Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations’. Elaborating further on this principle, the Commentary to Principle 1 Friendly Relations Declaration provides that

[e]very State has the duty to refrain from the threat or use of force to violate the existing international boundaries of another State or as a means of solving international disputes, including territorial disputes and problems concerning frontiers of States.

70 According to Principle III Helsinki Final Act (1975), entitled ‘Inviolability of Frontiers’,

[t]he participating States regard as inviolable all one another’s frontiers as well as the frontiers of all States in Europe and therefore they will refrain now and in the future from assaulting these frontiers. Accordingly, they will also refrain from any demand for, or act of, seizure and usurpation of part or all of the territory of any participating State.

G. Conclusion

71 The importance of boundaries in contemporary international law evidences the critical importance of their careful establishment. If the legal aspects of boundary making are important, the technical aspects matter no less. Boundary making by necessity involves, at each stage of the process, all the legal and technical expertise needed for the delimitation, demarcation, monumentation, and maintenance of ‘the imaginary lines on the surface of the earth which separate the territory of one state from that of another’ (Jennings and Watts [eds] Oppenheim’s International Law [1992] 661). If, during colonial periods, boundaries could be traced on territories based on inaccurate or even inexistent knowledge of the field, no reason would today justify drawing a boundary without prior investigation of the area to be delimited. A significant number of boundary disputes might have been obviated if the parties had, throughout the process, relied on accurate information. In the same vein, in almost every boundary dispute submitted to judicial or arbitral determination, the international court or tribunal has reached the conclusion that (at least with respect to one section, if not several sections, of the boundary line) the information provided by the disputing parties did not allow identifying the course of the boundary. The fact that the large majority of legal instruments that define boundaries tend to be at least a century old may justify this fact. International courts and tribunals do not, in other words, necessarily know better than the parties themselves what is the course of the boundary line. States should therefore strive to settle their boundary disputes primarily through direct and friendly means. As is well-known, ‘the judicial settlement of international disputes, with a view to which the Court has been established, is simply an alternative to the direct and friendly settlement of such disputes between the Parties’ (Free Zones of Upper Savoy and the District of Gex [France v Switzerland] [Order] [1929] at 13). Direct and friendly settlement of boundary disputes by the parties allows them to take into account fully all the interests and equities relating to their boundary, especially those of the population living in the boundary area, and together to give shape to effective solutions to common challenges, such as the impact of climate change on shared transboundary resources and cross-border criminality.

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Cited Documents

Cited Cases

Further Cases

Minquiers and Ecrehos (France/United Kingdom) [1953] ICJ Rep 47.

Boundaries

Eirik Bjorge, Mamadou Hébié

A. Introduction

B. Terminology

C. The Boundary-making Process in International Law

D. Establishing Boundaries

1. Boundary Agreements

2. Decisions of International Conferences or International Organizations Establishing Boundaries

3. The Creation of States and Boundaries

(a) Boundaries of States Created Through the Exercise of the Right of Peoples to Self-Determination

(b) Boundaries Resulting from the Creation of States in the Context of Secession

4. Maps

E. The Judicial Settlement of Boundary Disputes

1. The Impact of the Mandate of International Courts and Tribunals on the Settlement of Boundary Disputes Based on International Law

(a) Impact of Human Considerations

(b) Impact of Equitable Considerations

(c) Impact on the Doctrine of Natural Boundaries

2. The Relation Between Titles and Effectivités

(a) The Primacy of Titles over Effectivités

(b) Acquiescence

F. Fundamental Principles of International Law Applicable to Boundaries

1. The Principle of the Stability and Finality of Boundaries

2. The Principle of the Inviolability of Boundaries

G. Conclusion

Cited Bibliography

Further Bibliography

Cited Documents

Cited Cases

Further Cases

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Boundaries

Eirik Bjorge, Mamadou Hébié

A. Introduction

B. Terminology

C. The Boundary-making Process in International Law

D. Establishing Boundaries

1. Boundary Agreements

2. Decisions of International Conferences or International Organizations Establishing Boundaries

3. The Creation of States and Boundaries

(a) Boundaries of States Created Through the Exercise of the Right of Peoples to Self-Determination

(b) Boundaries Resulting from the Creation of States in the Context of Secession

4. Maps

E. The Judicial Settlement of Boundary Disputes

1. The Impact of the Mandate of International Courts and Tribunals on the Settlement of Boundary Disputes Based on International Law

(a) Impact of Human Considerations

(b) Impact of Equitable Considerations

(c) Impact on the Doctrine of Natural Boundaries

2. The Relation Between Titles and Effectivités

(a) The Primacy of Titles over Effectivités

(b) Acquiescence

F. Fundamental Principles of International Law Applicable to Boundaries

1. The Principle of the Stability and Finality of Boundaries

2. The Principle of the Inviolability of Boundaries

G. Conclusion

Cited Bibliography

Further Bibliography

Cited Documents

Cited Cases

Further Cases

Go to full text on: