A. A Note on Terminology
1. Property, Ownership, and Acquisition of Territory: Clarification of Concepts
1 The expression ‘acquisition of territory’ is usually employed as meaning the establishment of sovereignty over a given piece of land. Well-known UN Security Council resolutions refer to ‘acquisition of territory’ in this manner, notably Resolution 242 (1967). The expression, however, requires some precision. First, strictly speaking, ‘territory’ as a term of art comprises not only emerged land, but also airspace, the territorial sea, and internal waters (see Military and Paramilitary Activities in and against Nicaragua [Nicaragua v United States of America] [Merits]  ICJ Rep 14 at para. 212; Military and Paramilitary Activities in and against Nicaragua Case [Nicaragua v United States of America]). However, what is essentially at issue in the ‘acquisition of territory’ is the acquisition of (emerged) land, with the rest of the above-mentioned components of the legal notion of ‘territory’ being land appurtenances. Secondly, States acquire, or establish through certain acts and conduct, a particular territorial status or particular rights rather than ‘territory’ per se. As will be examined below, different legal statuses apply to territory including sovereignty, which is undoubtedly the most important and common, albeit not the only possible legal status.
2 In Sovereignty over Pedra Branca/Pulau Batu Puteh, Middle Rocks and South Ledge (Malaysia/Singapore), the International Court of Justice (ICJ) noted that the term ‘ownership’ is sometimes used to refer to sovereignty (at para. 222; Sovereignty over Pedra Branca/Pulau Batu Puteh, Middle Rocks and South Ledge Case [Malaysia/Singapore]). Indeed in State practice, expressions such as ‘acquisition of ownership over a territory’, ‘to possess the property of a territory’, or ‘to have dominium over a territory’ are often considered synonymous to ‘acquisition of territorial sovereignty’. Despite this confusing practice, a clear distinction must be maintained between the acquisition of private rights over a given territory and the acquisition of sovereignty over that territory. A decision concerning sovereignty over a given territory does not prejudice the ownership rights that individuals or even States may possess in the same territory. The ICJ chamber in Frontier Dispute (Benin/Niger) observed ‘that the question of the course of the boundary on the bridges is totally independent of that of the ownership of those structures, which belong to the Parties jointly’ (at para. 124; Frontier Dispute Case [Benin/Niger]). There are other examples of pieces of land that are under the sovereignty of one State and the property of another (the Mundat Forest, under German sovereignty and French ownership; the region of Tiwinza, under Peruvian sovereignty and Ecuadorian ownership). Besides, it is generally accepted that there can be a transfer of property without a transfer of sovereignty and vice-versa. Moreover, a transfer of sovereignty over a territory does not affect private property rights therein (see German Settlers in Poland [Advisory Opinion] 36 and 38; Certain German Interests in Polish Upper Silesia [Germany v Poland] [Merits] 22 and 42 [German Interests in Polish Upper Silesia Cases]; Land, Island and Maritime Frontier Dispute [El Salvador/Honduras: Nicaragua Intervening] [Merits] para. 66 [Land, Island and Maritime Frontier Dispute Case (El Salvador/Honduras: Nicaragua Intervening)]; Frontier Dispute [Benin/Niger] para. 118).
2. Modes of Acquisition of Territory and Titles to Territory
3 Traditionally, doctrine referred to the different means of acquiring territorial sovereignty as ‘modes of acquisition of territory’, by drawing an analogy with domestic law for the acquisition of ownership over land. This doctrinal construction was criticized for not being able to embrace all the various ways through which territorial sovereignty is established. At present, the notion of ‘title’ is preferred. It refers generally to the acts or facts that constitute the legal foundation for the establishment of a right over territory. The notion of title is often employed to refer either to the source of a right or to the proof of it (Frontier Dispute [Burkina Faso/Republic of Mali] [Judgment] para. 18; Frontier Dispute Case [Burkina Faso/Republic of Mali]). Maps provide a good example to illustrate this distinction. When annexed to a treaty of cession of sovereignty, maps have the same value as the treaty. They are an integral element of the relevant ‘title-source’, which is the treaty of cession. However, when maps are not annexed to such a treaty, they merely constitute evidence that a State may invoke in order to prove its title over the territory in dispute. Their probative value will depend, in these circumstances, on factors such as their accuracy, their official character, or the authority establishing them (ibid paras 54–55). Unless stated otherwise, this entry uses the term ‘title’ to mean the source of a right or competence over territory.
B. The Categorization of Territorial Titles
4 From the perspective of the rights and competencies bestowed, titles can confer sovereignty, administration, or private ownership over land. In so far as the existence of a title can be linked to the existence of a previous title, titles can be classified as original and derivative titles.
1. Title of Sovereignty and Title of Administration
5 Not all relations between a State and a territory can be explained by reference to the notion of sovereignty. Territorial sovereignty refers to the plenitude of a State’s competences over a territory. To possess sovereignty over a territory implies, according to Arbitrator Max Huber in the Island of Palmas Case (or Miangas) (Palmas Island Arbitration), that a State is legally entitled to exercise therein the plenitude and exclusivity of State competences. This includes its capacity to make decisions concerning the fate of the territory, ie to alienate it. In other circumstances, a State may administer a territory without being its sovereign. Consequently, assessing the nature of the relations between a State and a territory requires, first and foremost, to distinguish titles of sovereignty from titles of administration. States that exercised a mandate (Mandates) under Art. 22 League of Nations Covenant or a trusteeship under Chapter XII UN Charter (United Nations Charter; United Nations Trusteeship System) had a title to administer the territories on the basis of the agreements concluded with the League of Nations and the United Nations (UN) respectively, without having acquired sovereignty over the territories in question. A State may confer the exercise of authority over a given territory to another State, whilst retaining its sovereignty. This can also be the result of a decision of an international organization or a peace conference.
6 The difference between a title of sovereignty and a title of administration lies in the ability of its holder to dispose of the territory concerned. On the one hand, States enjoying a title of sovereignty can exercise the plenitude and exclusivity of rights related to a territory. On the other hand, States that simply administer a territory only possess the specific powers that are conferred on them by the title they hold, ie the mandate or trusteeship agreement, or by the treaty-based and customary rules governing the factual situation in which a State finds itself in control of a territory, such as the case of military occupation (Occupation, Belligerent).
2. Original and Derivative Titles of Sovereignty
7 The distinction between original and derivative titles is based on a traditional view about the modes of acquisition of territorial sovereignty. Traditionally, five modes are identified in legal literature as establishing territorial sovereignty, namely cession, effective occupation (Effectiveness), accretion, conquest, and prescription. The distinction between the original and derivative acquisition of sovereignty over territory served as a criterion to classify these modes. In territories characterized legally as terrae nullius, the acquisition of sovereignty is termed ‘original’, whereas for territories already under the sovereignty of another State, it will be referred to as ‘derivative’. On these lines, effective occupation of a terra nullius is considered as an original mode of acquisition of sovereignty, whereas cession is a derivative title of sovereignty. The ICJ endorsed this distinction when dealing with the agreements concluded between local chiefs and colonial powers in the Western Sahara Advisory Opinion. According to the Court, these agreements could not be considered as creating an original title, but were derivative titles of sovereignty (see Western Sahara [Advisory Opinion] paras 79–80; Western Sahara [Advisory Opinion]).
8 The distinction between original and derivative titles of territorial sovereignty has important legal consequences. On the one hand, the legal impact of possession by a State is not the same in the case of a terra nullius as it is in the case of a territory under a State’s sovereignty (see below the relations between titles and effectivités). On the other hand, concerning derivative titles of sovereignty, the validity of the transfer will depend on whether the previous sovereign possessed a lawful title over the territory purportedly ceded. Max Huber emphasized this aspect in Island of Palmas when he noted that
3. A Brief Overview of Titles of Sovereignty
9 One of the five modes of acquisition of territorial sovereignty identified in the legal literature is no longer a valid title (conquest, including debellatio). Another mode is essentially related to past periods of history (effective occupation, since it can generally be affirmed that there are no longer terrae nullius). The existence of another mode under international law is controversial (acquisitive prescription). Moreover, State practice and case law evince that there are other titles by which territorial sovereignty is established.
10 A title of sovereignty can be established or determined by treaty. This may be a peace treaty, a treaty of cession, or a treaty of delimitation. In addition, special mention should be made of the treaties concluded between colonial powers and local chiefs. The history of colonization reveals that there was a consistent practice of colonial powers entering into treaties with local chiefs for various purposes, including in order to transfer the latter’s territories to colonial powers. In this regard, the ICJ considered in the Western Sahara Advisory Opinion that ‘such agreements with local rulers, whether or not considered as an actual “cession” of the territory, were regarded as derivative roots of title, and not original titles obtained by occupation of terrae nullius’ (at para. 80). The exact scope of those treaties with regard to territorial sovereignty depends on their content (Maritime Delimitation and Territorial Questions between Qatar and Bahrain [Qatar v Bahrain] [Merits] paras 38–44, 95 [Maritime Delimitation and Territorial Questions between Qatar and Bahrain (Qatar v Bahrain)]; Land and Maritime Boundary between Cameroon and Nigeria [Cameroon v Nigeria: Equatorial Guinea Intervening] paras 200–9 [Land and Maritime Boundary between Cameroon and Nigeria Case (Cameroon v Nigeria)]; Sovereignty over Pedra Branca/Pulau Batu Puteh paras 135–36).
11 Attribution by an organ having the capacity to dispose of a given territory is also constitutive of title, either a title of sovereignty or of administration. It may be the decision of an international conference (see Memel Territory Statute, Interpretation of, Case), or of an international organization (see Treaty of Lausanne, Interpretation of [Advisory Opinion]) and UNGA Resolution 390 (V) concerning the federation between Ethiopia and Eritrea, or, in earlier times, a decision of the Pope through papal bulls (see Territory, Discovery). Decisions by tribunals having the capacity to settle ex aequo et bono a territorial dispute can also be constitutive of a title of sovereignty. As a matter of course, other decisions by adjudication (the ICJ or arbitral tribunals) have a declaratory character of what the legal existing situation is and, as such, they do not create territorial sovereignty (Frontier Dispute [Burkina Faso/Mali] para. 17).
12 The creation of a new State entails the establishment of a new sovereignty over the territory. Rules relating to the succession of States imply respect for existing boundaries, established by treaty or otherwise. The uti possidetis doctrine involves the transformation of former administrative limits into the boundaries of the new States (Frontier Dispute [Burkina Faso/Mali] paras 20–26; Land, Island and Maritime Frontier Dispute [El Salvador/Honduras] paras 40–43; Frontier Dispute [Benin/Niger] paras 23–25; Territorial and Maritime Dispute between Nicaragua and Honduras in the Caribbean Sea [Nicaragua v Honduras] paras 146–67 and 229–36 [Maritime Delimitation between Nicaragua and Honduras in the Caribbean Sea Case (Nicaragua v Honduras)]). As 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), uti possidetis was first applied in the context of accession of Latin American States to independence in the 19th century, then in the context of the decolonization of Africa, and, more recently, in the context of the creation of States in the territory of the Union of Soviet Socialist Republics and that of the Socialist Federal Republic of Yugoslavia (see, for instance, In the Matter of an Arbitration under the Arbitration Agreement between the Government of the Republic of Croatia and the Government of the Republic of Slovenia, Signed on 4 November 2009 [Final Award] [29 June 2017] PCA Case No 2012-04, para. 256 [‘Croatia v Slovenia’]). In Frontier Dispute (Burkina Faso/Niger), the Court confirmed its view, already expressed in Burkina Faso v Mali, that the principle of uti possidetis iuris and the ‘principle of the intangibility of boundaries inherited from colonization’, which is enshrined in the text of resolution AHG/Res 16(I) adopted in 1964 by the Organization of African Unity (‘OAU’), had the same legal significance (para. 63; Frontier Dispute Case [Burkina Faso/Niger]).
13 Unilateral acts can also constitute a title of sovereignty. Despite the controversy on whether the Permanent Court of International Justice (PCIJ) considered the Ihlen Declaration in Legal Status of Eastern Greenland (Denmark v Norway) (Eastern Greenland Case) as being a unilateral act or a tacit agreement, it is certain that unilateral acts can create obligations for States (Unilateral Acts of States in International Law; see also Tacit Agreements). As such, they can have the effect of transferring the sovereignty of a territory from one State to another. Unilateral acts can adopt the form of an explicit renunciation of territorial sovereignty and/or recognition that sovereignty belongs to another State. Acquiescence is a tacit expression of will manifested by unilateral conduct that can lead to the abandonment of sovereignty and its transfer to another State (Sovereignty over Pedra Branca/Pulau Batu Puteh paras 120–21).
14 The issue remains whether in a specific case a State can be considered to have acquiesced to the claim of an opposing party. The State that is alleged to have acquiesced should have direct or constructive knowledge of the acts accomplished by the State claiming to have acquired sovereignty through acquiescence. In addition, the acquiescing State must be under a duty to react to these acts. This will be the case when there is a clear claim of sovereignty over a territory by another State. In Temple of Preah Vihear (Cambodia v Thailand), the Court stressed both elements when it said:
15 As stated by the Court in Sovereignty over Pedra Branca/Pulau Batu Puteh, acquiescence should not be presumed lightly. In each case the clear consent to opposing claims needs to be established. Such consent can be tacit, implicit, or even construed. Nonetheless, it should be established beyond doubt. Admittedly,
In order to establish that a State has acquiesced to another State’s claims, judges will take into account the consistent conduct of the State regarding the opposing claims. Isolated acts will not be deemed as sufficient to create a title of territorial sovereignty based on acquiescence. In addition, State conduct, such as a protest or the way in which a State continues to treat in its domestic legal system the territory in dispute, may defeat any claim based on acquiescence.
4. Contested Approaches to Acquisition of Territorial Sovereignty
16 As mentioned above, the existence of acquisitive prescription in international law has been a matter of controversy since early times. In the second half of the 20th century, some scholars attempted to overcome this debate by putting forward the notion of historical consolidation of title. However, both the doctrine of acquisitive prescription and that of historical consolidation of title still remain controversial.
(a) Acquisitive Prescription
17 For its proponents, the doctrine of acquisitive prescription would be a title leading to the acquisition of territorial sovereignty where the territory in question had a previous holder of sovereignty, and where such acquisition occurs through the possession of a territory à titre de souverain over a prolonged lapse of time. However, neither the lapse of time nor prolonged possession, for this matter, which are the two characteristic elements of the concept of acquisitive prescription, are per se the legal bases justifying the transfer of sovereignty in a situation in which the territory at issue belongs to another State.
18 The role of the possession à titre de souverain will be discussed below. Regarding the time factor, it is worth recalling that there is no precise time limit prescribed by international law the lapse of which would be considered as conferring on a State in possession of a territory the right to acquire sovereignty thereof. In the Chamizal Case (Mexico/United States of America) (see American-Mexican Boundary Disputes and Cooperation), the arbitral award stressed the absence of a fixed deadline for this purpose (at 328). This view is undoubtedly correct as the proponents of the doctrine of acquisitive prescription tend to transpose into international law a domestic legal institution which needs for its implementation not only a fixed deadline, but also the authority of a judge to declare the transfer of title from its legitimate owner to the possessor, or its registration by a competent administrative body. However, nothing prevents States from establishing by treaty a deadline to prescribe, as was the case of the special agreement concluded between Great Britain and Venezuela on the Guyana Boundary Arbitration (Guyana-Venezuela Border Dispute). This constituted a unique case, leading to a decision based on a treaty, rather than on general international law.
19 It has been argued in academic literature that the role of acquisitive prescription, as a means of transferring title in international law, is fulfilled by other existing legal mechanisms at the international level. Indeed, the critical issue regarding transfer of territorial sovereignty in cases other than formal cession is not only the fact of possession à titre de souverain, but essentially the conduct of the dispossessed holder of title. As long as the title of the latter is not extinguished by abandonment (Territory, Abandonment), the possessor State cannot establish its sovereignty over the territory (Sovereignty over Pedra Branca/Pulau Batu Puteh paras 121–22). Therefore, aside from the unambiguous situation in which a State holder of the title formally renounces its sovereignty over a territory, acquiescence may play a decisive role for the transfer of sovereignty if all its conditions are met.
20 Acquisitive prescription has been discussed by the ICJ in territorial disputes, either because it was invoked by one of the parties as a principal or subsidiary title, or because one of the parties accused the other of claiming a prescriptive title (Minquiers and Ecrehos [France/United Kingdom]; Minquiers and Ecrehos Case; Temple of Preah Vihear; Sovereignty over Pedra Branca/Pulau Batu Puteh). In Kasikili/Sedudu Island (Botswana/Namibia), the Court stressed that the parties had agreed, in the compromis seizing the Court of the dispute, that acquisitive prescription is recognized in international law, as well as on the conditions under which a title of territorial sovereignty may be acquired by prescription. However, the judgment then stated that ‘the Court need not concern itself with the status of acquisitive prescription in international law or with the conditions for acquiring title to territory by prescription. It considers … that the conditions cited by Namibia itself are not satisfied in this case and that Namibia’s argument on acquisitive prescription therefore cannot be accepted’ (para. 97; Kasikili/Sedudu Island Case [Botswana/Namibia]). In Land and Maritime Boundary between Cameroon and Nigeria, the Court responded to Nigeria’s allegation that, even if Cameroon’s sovereignty was to be upheld, Nigeria still possessed a title of sovereignty arising from the long and peaceful exercise of the sovereignty over Bakassi. The Court emphasized the fact that Nigeria had never had a lawful title of sovereignty over this territory. Accordingly, the Court held that the effectivités relied upon by Nigeria were contradictory to law and could not, therefore, create a valid title of sovereignty for this country (at para. 64). In Sovereignty over Pedra Branca/Pulau Batu Puteh, the Court found that the transfer of sovereignty, ie the loss of sovereignty by Johor/Malaysia and its acquisition by the United Kingdom/Singapore, was due to the conduct à titre de souverain of the latter, and the conduct of the former which included the failure of Johor/Malaysia to respond to the British/Singaporean conduct.
21 A survey of State practice reveals that States are careful not to rely on acquisitive prescription as the main basis of their claims to territorial sovereignty. In fact, claims based on acquisitive prescription have the undermining effect of asserting that the original title belonged to the other State. Finally, case law shows that tribunals, rather than examining whether title was acquired through the ‘doctrine of prescription’ (as it was called by the Court in Kasikili/Sedudu Island para. 90), focus on State conduct by both sides to the dispute in order to determine whether there has been an abandonment of title and recognition of the other State’s sovereignty through acquiescence.
(b) Historical Consolidation of Title
22 In order to circumvent the theoretical and practical objections against acquisitive prescription some scholars proposed, in the second half of the 20th century, the notion of ‘historical consolidation of title’ as a title of territorial sovereignty (De Visscher; Schwarzenberger; Blum). Whereas acquisitive prescription is primarily based on the prolonged lapse of time during which a State held possession of a territory, the notion of historical consolidation of title focuses rather on the special interest a State may have in a given territory, and the general tolerance or recognition by other States of this claim. This notion was grounded on the dictum of the Court in Fisheries (United Kingdom v Norway) ( ICJ Rep 116 [Fisheries Case (United Kingdom v Norway)]) where the Court dealt with the claim of Norway to extend its fisheries zone beyond the limits recognized by international law (Fishery Zones and Limits).
23 In this case, the Court laid great emphasis on the attitude of other States vis-à-vis the Norwegian delimitation system and the benefit of general toleration. In Land and Maritime Boundary between Cameroon and Nigeria, the latter invoked historical consolidation of title in order to claim sovereignty over the Bakassi Peninsula. The Court stressed that ‘the notion of historical consolidation has never been used as a basis of title in other territorial disputes, whether in its own or in other case law’. For the Court, ‘the theory of historical consolidation is highly controversial and cannot replace the established modes of acquisition of title under international law, which take into account many other important variables of fact and law’ (at para 65). Crucially, the Court stressed that where a title of sovereignty exists, effectivités contra legem cannot prevail over it.
D. Role of Fundamental Principles of International Law
41 A title to territory or a claim to such a title does not exist in isolation from the international legal system. As such, both can be influenced by other rules and principles of international law which are relevant to this field. This is the case in particular for fundamental principles of international law. Contemporary principles such as the right to self-determination and the prohibition of the use of force have produced significant changes in the law relating to the establishment of territorial sovereignty.
1. Principle of Territorial Integrity
42 Respect for territorial integrity is a cornerstone principle of the contemporary international legal system. As the Court held it in the Corfu Channel case, ‘[b]etween independent States, respect for territorial sovereignty is an essential foundation of international relations’ ([Merits] 35; Corfu Channel Case). More recently, the Court recalled that ‘the principle of territorial integrity is an important part of the international legal order’ (Accordance with International Law of the Unilateral Declaration of Independence in Respect of Kosovo [Advisory Opinion] para. 80; Kosovo [Advisory Opinion]).
43 The international law regime governing the acquisition of territorial sovereignty attaches far-reaching consequences to the illegal character of acts performed in violation of a State’s territorial integrity (Territorial Integrity and Political Independence): a territory possessed as a result of a violation of the principle of territorial integrity cannot be considered as belonging to the possessor. Indeed, such a possession will be tainted with illegality. Consequently, the principle of territorial integrity strengthens the view that, for territories under State sovereignty, only consent—either expressed implicitly through acquiescence, or explicitly by a formal act like the conclusion of a treaty—can transfer the territorial title.
44 In the Chagos Advisory Opinion, the ICJ considered that Administering Powers should respect the territorial integrity of non-self-governing territories. Consequently, it considered that the separation of the Chagos Archipelago from Mauritius in 1965 was illegal (Chagos [Advisory Opinion]).
2. Principle of Self-determination
45 Self-determination is the most revolutionary of the principles enshrined in the UN Charter, since for the first time in human history it grants to a specific community, a ‘people’ (as this term is understood in international law), the right to modify territorial sovereignty through the change of the status of the territory upon which self-determination is exercised. This occurs through the creation of a newly independent State, or its integration or free association with a new or an existing one. In the Chagos Advisory Opinion, the ICJ clarified that the right of self-determination had already acquired a customary international law character in 1960. For the Court, Resolution 1514 (XV) of the General Assembly had a ‘declaratory character with regard to the right to self-determination as a customary norm’ (para. 152).
46 The principle of self-determination produces two major effects as far as territorial sovereignty is concerned.
47 First, the principle has an impact on territorial titles formerly held by colonial powers. By virtue of the application of the rules of intertemporal law, colonial titles can no longer be maintained as they are at variance with the right of self-determination. With the consecration of the right of self-determination as a principle of international law, the titles of sovereignty held by former colonial powers were transformed into titles of administration. The Friendly Relations Declaration (1970) appended to UNGA Resolution 2625 (XXV) consecrated this transformation of colonial titles by asserting that ‘[t]he territory of a colony or other Non-Self-Governing Territory has, under the Charter, a status separate and distinct from the territory of the State administering it’.
48 Second, the right to self-determination provides the basis for the sovereignty of ‘peoples’ over their territory (Peoples). Resolution 2625 even provides that ‘peoples’ have a right to territorial integrity when it declares that ‘[e]very State shall refrain from any action aimed at the partial or total disruption of the national unity and territorial integrity of any other State or country’. In the Chagos Advisory Opinion, the Court linked the right to self-determination to the obligation of respect for territorial integrity. Accordingly, the Court held that
49 It is crucial to define the notion of ‘people’ in order to determine what are the human groups that can claim a right to exercise self-determination under international law. As the Court observed in the Western Sahara Advisory Opinion, not every population inhabiting a territory constitutes a ‘people’ entitled to self-determination (at para. 59). This was reiterated in the Chagos Advisory Opinion (at para. 158). In the Kosovo Advisory Opinion, the Court explained that ‘[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’ (para. 79). The Court distinguished declarations of independence made in the context of the exercise of the right of self-determination from declarations of independence made ‘outside this context’ (ibid).
50 The practice of the UN shows that within the Organization this right was applied only to peoples living under colonial or alien domination. Existing States are considered to be constituted by a single people, unless the State itself recognizes that it comprises a plurality of peoples entitled to exercise self-determination. If understood in this manner, it is possible to reconcile the right to exercise self-determination with the principle of territorial integrity. However, there is debate concerning the scope of the seventh paragraph dealing with self-determination in the Friendly Relations Declaration, which contains the ‘saving clause’ of territorial integrity of States (see Secession).
3. Principle of the Prohibition on the Use of Force
51 The inclusion of a general prohibition on the use of force in international relations by the UN Charter has had a considerable impact on the international law regime governing territorial change (Use of Force, Prohibition of). In the Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory (Advisory Opinion), the Court stressed the connection between these two rules. It held that the international rule prohibiting the use of force in international relations and ‘its corollary’, the international law rule prohibiting the acquisition of territorial sovereignty resulting from the threat or use of force, were customary rules of international law (para. 87; Israeli Wall Advisory Opinion [Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory]). Drawing upon a large international practice dating from the First Pan-American Conference of 1890 and the Stimson Declaration, Resolution 2625 consecrated the duty bearing upon all States not to recognize territorial situations established in violation of the prohibition on the use or threat of use of force. It declared that ‘[n]o territorial acquisition resulting from the threat or use of force shall be recognized as legal’ (Friendly Relations Declaration; see also the definition of aggression embodied in UNGA Resolution 3314 [XXIX]). Consequently, conquest and debellatio, which used to be, under certain conditions, lawful means of acquisition of territorial sovereignty, are no longer valid titles of territorial sovereignty in contemporary international law.
E. Technical Rules in Territorial Disputes
52 Technical rules such as the critical date and the intertemporal law rule do not exclusively operate in the field of acquisition of territorial sovereignty, although they play a significant role in this area.
1. Critical Date
53 Territorial claims imply a succession of events occurring over a considerable span of time. A tribunal is therefore obliged to determine the moment in time when the claims submitted by the parties to a dispute must be legally assessed. This moment in time is the critical date. The determination of the critical date may have a significant impact on both the applicable law and the facts considered to be relevant to the dispute. In Sovereignty over Pedra Branca/Pulau Batu Puteh, the Court considered that the critical date serves to ‘distinguish … between those acts which should be taken into consideration for the purpose of establishing or ascertaining sovereignty and those acts occurring after such date’ (at para. 32).
54 In practice, facts occurring after the critical date will not be taken into account to determine the existence of a title of sovereignty. A State party to a territorial dispute may attempt to improve its legal position by performing self-serving acts. Exceptionally, international tribunals have taken into account some facts that occurred after the critical date. These facts have the peculiar feature of being the continuation of activities previously undertaken by the States concerned before the dispute, and do not improve their legal position (ibid para. 32). Indeed, what is essential is the probative value of such acts, especially in light of their coherence with positions held before the critical date. In particular, acts performed after the critical date cannot be invoked as an independent source of title. This is of particular relevance with regard to acquiescence.
55 In general, the critical date is the date when the dispute crystallizes, ie when the parties formally opposed each other’s claim. This generally occurs when one side asserts its sovereignty and the other side protests for the first time, or when the first protest by one State is rejected by the other (ibid para. 33). However, there can be particular critical dates depending on the titles claimed by the parties to a dispute. For disputes relating to uti possidetis, the critical date will be the date of accession to independence, in order to determine the existing situation that was inherited by the successor States concerned.
2. Intertemporal Law
56 The intertemporal law rule plays an important role in addressing the effect of the evolution of the international legal system on territorial titles. According to Max Huber,
Thus, to determine whether a State acquired sovereignty over a territory in the 19th century, the relevant rules of international law that existed at that period must be applied. However, determining whether sovereignty is still maintained nowadays requires examining whether it still complies with the rules of international law that have since evolved. In this manner, the first part of the rule of intertemporal law applies only to acts and facts that give rise to a title of sovereignty, whereas the second part of intertemporal law applies to the legal situation established by this title.
57 Sovereignty acquired by conquest in the 18th or 19th centuries, for example, must be analysed on the basis of the conditions set down by international law for such an acquisition at that time, and not by applying the rules on the prohibition of the use of force existing in contemporary international law. However, the maintenance of sovereignty must conform to the present legal situation. Consequently, sovereignty acquired over a territory in conformity with the international legal system existing at the time of the acquisition can be superseded by the evolution of international law. For instance, the recognition of the existence of a people entitled to exercise the right of self-determination over a territory would render any title of sovereignty formerly held on that territory obsolete (caduc).