Preemptive war occurs when a state that is about to be attacked decides to strike first at its enemy and thus disrupt the impending attack. Unlike preventive war, in which a state strikes a potential enemy even during a time of relative peace (an action that is usually inadmissible under international law), a preemptive war takes place when a state is under the direct threat of imminent hostilities. Preemptive war is therefore a form of self-defense, and is generally accepted as a legitimate use of force under international law.

A modern example of preemptive war occurred in 1967 at the start of the so-called Six-Day-War between Israel and its Arab neighbors. On May 15, 1967, Egypt and Syria, after weeks of intensifying tensions with Israel, concentrated large numbers of military forces on Israel's border. The leader of Egypt, Gamal Abdel Nasser, proclaimed his readiness to make war against Israel...