War, Declaration of, formal announcement of hostile intentions by one country to another. In most nations, the power to declare war belongs to the executive or sovereign. The United States president may recognize a “state of war” initiated against the United States by a foreign power or by domestic insurgents.
II/. DECLARATIONS PRECEDING HOSTILITIES
The practice of making a declaration before actually committing an act of war originated in ancient Greece and Rome. During feudal times, in the Middle Ages, heralds were sent to warn the enemy of impending hostilities. Such declarations of war were usual until the early 18th century. One of the best-known examples in more recent times was the French declaration of war against Prussia several weeks before the French launched the first attack in the Franco-Prussian War (1870-1871).
The Second Hague Conference—the international peace conference that met in the Netherlands in 1907—accepted a convention asserting that hostilities between nations might not be undertaken without a prior declaration; this convention was largely ignored, however, after World War I ended in 1918. According to the charter of the United Nations, all nations are to refrain from the use of force in the settlement of disputes. Nations engaged in conflict have therefore attempted to avoid the charge that they are at war by avoiding formal declarations.
III/. DECLARATIONS AFTER HOSTILITIES HAVE COMMENCED
Declarations after hostilities have commenced have sometimes been made by belligerents in modern times as a means of justifying their actions in international law and world opinion. Diplomatic exchanges usually precede such declarations and, after hostilities have begun, both parties frequently assert that they are acting in self-defence. In some cases, a nation presents an ultimatum, or conditional declaration of war, stating that unless a favourable reply to its terms is received within a certain time, war will commence.
Sometimes a sudden attack by one nation upon another takes place without ultimatum or declaration of war, as in the case of the Japanese attack on the US naval base at Pearl Harbor, Hawaii, on December 7, 1941. The declaration of war against Japan by which the United States entered World War II was made by the Congress of the United States the following day.