Until 1945, an act of war in the traditional and historical sense was understood to mean any act by a State that would effectively terminate the normal international law of peacetime and activate the international law of war. The decision was invariably that of the target State and was generally preceded by a statement warning that certain acts would be considered acts of war and would trigger hostilities. Belligerent and neutral States also used the term. Belligerents would interpret as acts of war any action that seemed to assist the enemy; neutrals, any infringement of their neutrality.
In 1945, the United Nations Charter banned the first use of force, putting an end to declarations of war. "All members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any State," states Article 2 of the charter. The last declaration of war was made by the Soviet Union against Japan in 1945. An example of modern State practice is provided by the United Kingdom, which during the Suez War of 1956 and the Falkland Islands War of 1982 strenuously denied that it was at war with, respectively, Egypt and Argentina. Britain applied the laws of armed conflict in its military operations, nevertheless.
The term act of aggression has to all intents and purposes subsumed the legal term act of war and made it irrelevant, although act of war is still used rhetorically by States that feel threatened. The People's Republic of China stated in 1997 that any attempt by the Republic of China (Taiwan) to declare independence would be regarded as an act of war; and in August 1998, the U.S. Secretary of State, Madeleine Albright, said Osama bin Laden, the reputed mastermind of truck-bomb attacks on two U.S. embassies in Africa, had "declared war on the United States and struck first." In the domestic law of many States, act of war is also used in some contexts, such as insurance and reparations claims, to refer to any use of force in any armed conflict.
David Turns is a barrister and lecturer in law at the University of Liverpool in England. He is the editor of International Law and Espionage (Martinus Nijhoff, 1995, begun by the late Dr. J. Kish).