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Max Planck Encyclopedia of Public International Law [MPEPIL]


Marcelo G Kohen

From: Oxford Public International Law (http://opil.ouplaw.com). (c) Oxford University Press, 2023. All Rights Reserved.date: 28 January 2023

Peace treaties — Belligerence — Occupation — Self-defence — Conduct of hostilities — Aggression

Published under the auspices of the Max Planck Institute for Comparative Public Law and International Law under the direction of Professor Anne Peters (2021–) and Professor Rüdiger Wolfrum (2004–2020). 

A.  Notion

Conquest is a concept of classical international law. Its relevance in contemporary international law will be discussed later in this entry. The Permanent Court of International Justice (‘PCIJ’) defined conquest as ‘a cause of loss of sovereignty when there is war between two States and by reason of the defeat of one of them sovereignty over territory passes from the loser to the victorious State’ (Eastern Greenland Case at 47). Conquest, strictly so-called, is therefore a derivative title of acquisition of territorial sovereignty (Territory, Acquisition) taking place in the framework of war. As such, its existence is conditioned on the evolution of international law with regard to the use of force.

The seizure of the enemy’s territory as a result of war does not automatically imply a transfer of sovereignty to the victor. In some cases, the victorious State is not willing to produce such a change. Even where the victor wilfully sought to acquire sovereignty, the result could not be produced unilaterally. For conquest to result in a valid acquisition of territory, international law required a number of conditions. Before addressing these conditions, the differences between conquest and other situations must first be explained.

B.  Distinction with Other Territorial Statutes Having Their Origin in War

Conquest has often been confused with other territorial situations. First of all, it must be distinguished from belligerent or military occupation (Occupation, Belligerent; Occupation, Military, Termination of). Mere control of the enemy territory at the end of hostilities amounts only to a situation of military or belligerent occupation. This is a purely factual situation, essentially governed by the law of armed conflict, in which the occupier exercises most of the State functions but only as an administrator, whereas the population concerned is protected by particular requirements of humanitarian law (Humanitarian Law, International). Particularly, sovereignty remains unchanged during the period of belligerent or military occupation.

Conquest and annexation are not synonymous either. The latter term is used within and outside the context of armed conflicts, to designate a unilateral decision adopted by a State in order to extent its sovereignty over a given territory. In many cases, the effective occupation of a terra nullius was followed by a declaration of annexation, in order to incorporate the territory under the sovereignty of the acquiring State. In the context of armed conflicts, annexation is the case in which the victorious State unilaterally declares that it is henceforth sovereign over the territory having passed under its control as a result of hostilities. This attempt at producing a transfer of sovereignty through the exclusive decision of the victor is not generally recognized as valid, both in classical and in contemporary international law. An example of a case of annexation preceding the adoption of the UN Charter is the annexation of Bosnia-Herzegovina by the Austro-Hungarian Empire in 1908. The annexation was not recognized by the major powers and required a modification of the 1878 Treaty of Berlin (Berlin Congress [1878]), which had simply granted Austria-Hungary the right to administer the territory (Treaty between Austria-Hungary, France, Germany, Great Britain, Italy, Russia, and Turkey for the Settlement of Affairs in the East [signed 13 July 1878] 153 CTS 171). Another example is the annexation of Ethiopia by Italy in 1936. Examples of purported contemporary annexations are the Golan Heights annexed by Israel in 1980 and Kuwait by Iraq in 1990, both declared null and void by the Security Council (Iraq-Kuwait War [1990–91]), or the incorporation of Crimea and the City of Sebastopol in the Russian Federation.

Conquest as an acquisition of part of the territory of the enemy must also be distinguished from debellatio, also called subjugation. In the latter case, the defeated State ceases to exist as a result of the war and the victorious State is in a position to acquire sovereignty on the basis of its own exclusive decision. Since there is no subject with whom to conclude a peace treaty, the victorious State could ultimately decide the fate of the defeated State.

C.  Conditions for the Existence of Conquest as a Valid Title to Territory

At the time when conquest was a valid title of acquisition of territorial sovereignty, it was subordinated to the fulfilment of three particular conditions. The material element was the effective control over the seized territory after the close of hostilities. In addition, a previous declaration of war establishing the existence of a state of war among the belligerents was a formal premise. A treaty of peace putting an end to that state of war and endorsing the transfer of sovereignty was perceived as a third decisive condition (Peace Treaties).

Some authors, like Lord McNair, Ernest Nys, and Charles Rousseau, considered that the Treaty of Utrecht of 1713 (Treaty of Peace and Friendship between Great Britain and Spain [signed 13 July 1713, entered into force 4 August 1713] 28 CTS 295) marked the beginning of the practice requiring a peace treaty as necessary for the acquisition of territorial sovereignty in the context of conquest, whereas others, such as Paul Fauchille, were of the view that this requirement only emerged in 1815. The requirement of a peace treaty as an essential condition for the operation of a transfer of sovereignty was recognized and applied in case-law (see, for example, the Ottoman Debt Arbitration and the Central Rhodope Forest arbitral awards and the Lighthouses in Crete and Samos case before the PCIJ).

Jurisprudential scholarship on the issue indicates a controversy about whether it is the conquest or rather the treaty of peace that is the root of territorial title. For some authors, like Robert Jennings, the existence of such a treaty renders the case as one of cession. Consequently, there would be no need to refer to conquest as a distinct title of acquisition of territorial sovereignty. In this view, the term ‘conquest’ would only be reserved for a situation of military territorial seizure followed by annexation. This position amounts to accepting that the conqueror can unilaterally decide the fate of the territory seized from the defeated State, and it blurs the distinction between conquest and annexation. Authors holding this view recognize that the situations of purported transfer of territory without a peace treaty were rather exceptional. Conquest can indeed be distinguished from mere cession since it was a root of title based in a composite set of elements in the context of the existence of a state of war, the peace treaty being the final act of this process. In contrast, cession operates as a derivative title in times of peace, in an entirely different context, in which coercion plays no role.

D.  Its Existence under Classical International Law

Regardless of the existence of early doctrinal divergence as to the morality of territorial acquisition through war, there is no doubt that conquest was a valid title of acquisition of territorial sovereignty during the time period when international law recognized war as a legitimate way of pursuing national interests.

10  At that time, treaties obtained by coercion of the defeated State were not considered void. Consequently, the whole operation of a declaration of war, the consequent occupation of foreign territory with the intention of acquiring sovereignty, and a peace treaty affecting a transfer of sovereignty was a valid and recognised mode of acquisition of title. This means of acquiring sovereignty was applicable among European States themselves as well as with regard to overseas territories possessing some structured organization of power.

E.  Its Prohibition in Contemporary International Law

11  Conquest as a valid mode of acquisition of territory began to be outlawed on a regional basis, beginning in the Americas during the 19th century. Both the uti possidetis doctrine and the Monroe doctrine had the purpose of avoiding forcible territorial changes in the hemisphere (Doctrines [Monroe, Hallstein, Brezhnev, Stimson]). The First Inter-American Conference adopted a Resolution on the ‘Right of Conquest’ on 18 April 1890, declaring that ‘the principle of conquest shall not…be recognized as admissible under American public law’. Subsequently, other declarations and treaties were adopted in the region prohibiting war and conquest, particularly the Anti-War Pact of Rio of 10 October 1933, known also as the Saavedra Lamas Treaty (1933) or Argentine Pact (163 LNTS 393).

12  It is difficult to determine at what precise moment conquest ceased to be a valid mode of acquisition of territorial sovereignty in general international law. Art. 10 Covenant of the League of Nations played an important role by prohibiting aggression and promoting the respect of the territorial integrity of the Member States. The Pact of Paris (Kellogg-Briand Pact [1928]) condemning recourse to war and renouncing it as ‘an instrument of national policy’ continued this evolution (94 LNTS 57). These instruments favoured the condemnation of direct or concealed attempts at annexation, such as that of Italy with regard to Ethiopia and that of Japan over Manchuria. The prohibition of the threat or the use of force set out in Art. 2 (4) UN Charter marked the achievement of that evolution (Use of Force, Prohibition of Threat). Resort to force being illegal, there is currently no possibility of producing a territorial change of sovereignty as a result of it. Nor can the exception to that prohibition in the case of self-defence be validly invoked as a justification of conquest, since its aim is limited to expelling the aggressor, and does not extend to the enlargement of one’s territory.

13  The outlawing of conquest is referred to in many instruments at different levels. The Friendly Relations Declaration (1970) states that ‘[t]he territory of a State shall not be the object of acquisition by another State resulting from the threat or use of force’. Art. 5 (3) UNGA (Definition of Aggression declares that ‘[n]o territorial acquisition or special advantage resulting from aggression is or shall be recognized as lawful’ (UNGA Res 3314 (XXIX) (14 December 1974)) GAOR 29th Session Supp 31 vol 1, 142). Several Security Council and General Assembly resolutions on particular situations have also recalled ‘the inadmissibility of the acquisition of territory by war’, or used equivalent wordings (see Middle East [UNSC Res 242], Bosnia and Herzegovina [UNSC Res 859], Nagorny-Karabakh [UNSC Res 874], Georgia [UNSC Res 896]), and Ukraine [UNGA Res 68/262]).

14  It has also been asserted that in contemporary international law, conquest would be contrary to the principle of self-determination, since the destiny of a given territory is determined by the people concerned and not by resort to arms. At any rate, any seizure of the territory over which a people still have to exercise their right to self-determination could be considered as an obstacle to that exercise, and a violation of the territorial integrity of the self-determination unit.

15  The law therefore evolved over the course of the 20th century, allowing Judge Philip Jessup to state that ‘[i]t is a commonplace that international law does not recognize military conquest as a source of title’ (South West Africa Cases [Ethiopia v South Africa; Liberia v South Africa] [Second Phase] [1966] ICJ Rep 6, 418).

F.  Special Problems

16  According to the intertemporal law, forcible territorial changes operating in conformity with international law at the time when conquest was a recognized rule of international law cannot be challenged at present. It is likely that the last examples of acquisition of territorial sovereignty as a result of war are those former German territories east of the Oder-Neisse Line and Königsberg, which passed to Poland and the Soviet Union at the end of World War II. Although no peace treaty determined the explicit transfer of sovereignty, the defeated State abandoned its sovereignty in other ways, including the conventional recognition of the Oder-Neisse Line as the boundary with Poland and its declaration that it had no territorial claims against the Soviet Union and Poland (see Treaty between the Federal Republic of Germany and the Soviet Union [signed 12 August 1970]; Agreement between the Polish People’s Republic and the Federal Republic of Germany Concerning the Basis for Normalization of their Mutual Relations [signed 7 December 1970, entered into force 3 June 1972] 830 UNTS 327; Treaty on the Final Settlement with Respect to Germany [signed 12 September 1990, entered into force 15 March 1991] 1696 UNTS 115). Another example is the case of the Southern part of Sakhalin and the Kuril Islands, sovereignty over which was renounced by Japan at the San Francisco Peace Treaty of 8 September 1951 (136 UNTS 45; Peace Treaty with Japan [1951]).

17  A further question is whether a forcible change of a territorial situation occurring in contemporary times can ultimately lead to a transfer of territorial sovereignty. In principle, the answer is negative, since such an act would be contrary to the fundamental principle of the prohibition of the use of force. This does not necessarily mean that transfer of territorial sovereignty cannot be achieved through the application of other rules. The case of Goa and other Portuguese enclaves in India is exemplary (Goa, Conflict), since it involved the invocation of the principle of self-determination and the further retroactive recognition of Indian sovereignty by Portugal through the treaty of 31 December 1974 (982 UNTS 158).

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