Until 1945, an act of war in the traditional, 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. Article 2(4) of the Charter states: “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.” The last formal declaration of war was made by the U.S.S.R. against Japan in 1945. An example of modern State practice is provided by the United Kingdom, which during the Suez War (1956) and the Falkland Islands War (1982) strenuously denied that it was at war with, respectively, Egypt and Argentina. Britain applied the law of international 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 that 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.

Following the terrorist attacks on the World Trade Center and the Pentagon on 11 September 2001, U.S. President George W. Bush declared that America had been attacked and that the terrorists’ actions constituted “acts of war” (they were also implicitly recognized by the UN Security Council as giving rise to a right of self-defense, albeit against a non-State group rather than a State). This was followed by the U.S. Congress passing a Resolution to give war powers to the President for the prosecution of the “war.” Even so, use of the term “acts of war” to describe the terrorist attacks of 9/11 is, in legal terms, fairly meaningless—acts of war are committed by sovereign States—and it should be viewed, again, as primarily rhetorical in nature.

(See terrorism)