Welcome to my brief primers prepared for my courses at the University of Ottawa, Faculty of Law. My name is Craig Forcese. In this primer, we discuss territorial acquisition by states.
The acquisition of territory by one state from another has occurred through various means over the course of history. Some of these methods, once considered legitimate, are now prohibited under international law. This discussion focuses on two such means: one that has been rendered unlawful and one that remains lawful, though subject to conditions.
Conquest and Annexation: A Prohibited Practice
Historically, conquest followed by annexation was a primary method of acquiring territory. This involved the use of military force during wartime to seize enemy territory. However, conquest alone did not suffice; it was typically formalized through annexation, whereby the vanquished state’s existence or claim to sovereignty was eliminated. While such practices were accepted in earlier eras and their impacts continue to the extent that past conquests and annexations are not retrospectively unravelled, they are now unequivocally outlawed.
The prohibition of territorial acquisition by conquest began during the interwar period. A key milestone was the 1928 General Treaty for the Renunciation of War as an Instrument of National Policy, also known as the Kellogg-Briand Pact. This treaty declared that its parties renounced war as a means of resolving international disputes and committed to peaceful methods of conflict resolution. Although the treaty lacked enforcement mechanisms, it marked a significant shift in international norms.
After the Second World War, the UN Charter codified a prohibition of the threat or use of force in international relations in Article 2(4). Thus, except where authorized by the UN Security Council or in self-defence, a state may not use or threaten force against another state’s territorial integrity or political independence or otherwise in violation of the purposes of the United Nations. These purposes include friendly relations among nations based on respect for the principle of equal rights and self-determination of peoples.
The effect is to bar conquest. As the International Court of Justice has observed, “the prohibition of territorial acquisition resulting from the threat or use of force, as a corollary of the prohibition of the threat or use of force, is a principle of customary international law.” Thus, “occupation can under no circumstances serve as the source of title to territory or justify its acquisition by the occupying Power.” Further, “the prohibition of the acquisition of territory by force entails that the use of force is not a means for resolving claims of sovereignty.” The UN Security Council and the UN General Assembly have also reaffirmed that, under the Charter, states may not acquire territory by conquest.
The prohibition on territorial conquest entails specific legal obligations for all states. States must refrain from recognizing territorial acquisitions that result from unlawful uses of force. They are also prohibited from providing assistance that sustains such illegal situations.
Violations of the prohibition on conquest are not merely breaches of international law; they constitute serious international crimes. The International Military Tribunal at Nuremberg, established after the Second World War, prosecuted Nazi leaders for crimes against peace, now commonly referred to as the crime of aggression.
Under contemporary international law, the crime of aggression involves the planning, preparation, initiation, or execution of acts of aggression in manifest violation of the UN Charter. Such acts of aggression include the invasion or attack of one state’s territory by another’s armed forces, military occupation resulting from such actions, and annexation achieved through force.
Importantly, even if annexation follows a peace treaty, it remains unlawful if the treaty itself was procured through force. Treaties that violate the UN Charter through the threat or use of force are void under international law. Therefore, any territorial transfers arising from such treaties have no legal effect.
Cession: A Lawful, Conditional Alternative
In contrast to conquest and annexation, cession remains a lawful means of acquiring territory, provided it is conducted in accordance with international law. Cession occurs when one state transfers territory to another through mutual agreement, typically formalized in a treaty. For example, historical instances of cession include the Louisiana Purchase and the transfer of Alaska from Russia to the United States.
However, even treaties of cession are subject to scrutiny. If a treaty is concluded under coercion of a state representative or, as noted, the threat or use of force, it is invalid. Thus, while cession is a legitimate mechanism for territorial acquisition, it must not be engineered by a threat or use of force or coercion of state officials.
The Effectiveness of Modern Rules
It is true that some states have attempted to acquire territory through force in defiance of these rules. Nevertheless, such instances are relatively rare compared to the widespread territorial conquests of earlier centuries.
In their book The Internationalists, Oona Hathaway and Scott Shapiro note that between 1929 to 1948, successful conquests declined by 86% relative to the period 1816 to 1928. Even more notably, between the post-War territorial re-orderings ending in 1948 and the early 21st century, the amount of territory successfully acquired through forcible annexation was only 6% of the territory so acquired between 1816 and 1928.
Conclusion
The outlawing of conquest as a legitimate means of acquiring territory represents a fundamental achievement of the post-Second World War international legal order. These norms form the bedrock of a rules-based international order aimed at fostering peace, stability, and cooperation among states. That is why, unlike a large number of our ancestors, most people alive today have never experienced a conquest.
Thank you for your interest.
References
John Currie, Craig Forcese, Joanna Harrington, and Valerie Oosterveld, International Law: Doctrine, Practice, and Theory (Irwin Law, 2022, 3d Ed).
Oona Hathaway and Scott Shapiro, The Internationalists (Simon and Schuster, 2017).
John Norton Moore (chair), The Virginia-Georgetown Manual on the Law Concerning the Use of Force: Rules and Commentaries on Jus ad Bellum (West Point Press, 2024).
