A preemptive war is a war that is commenced in an attempt to repel or defeat a perceived imminent offensive or invasion, or to gain a strategic advantage in an impending (allegedly unavoidable) war shortly before that attack materializes. It is a war that preemptively 'breaks the peace' before an impending attack occurs.
Preemptive war is sometimes confused with preventive war: the difference is that a preventive war is launched to destroy the potential threat of the targeted party, when an attack by that party is not imminent or known to be planned. The U.S. Department of Defense defines a preventive war as an armed conflict "initiated in the belief that military conflict, while not imminent, is inevitable, and that to delay would involve greater risk." A preemptive war is launched in anticipation of immediate aggression by another party. Most contemporary scholarship equates preventive war with aggression, and therefore argues that it is illegitimate. The waging of a preemptive war has less stigma attached than does the waging of a preventive war.
