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'.
The term 'preemptive war' is sometimes confused with the term '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. 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.
The initiation of armed conflict: that is being the first to 'break the peace' when no 'armed attack' has yet occurred, is not permitted by the UN Charter, unless authorized by the UN Security Council as an enforcement action. Some authors have claimed that when a presumed adversary first appears to be beginning confirmable preparations for a possible future attack, but has not yet actually attacked, that the attack has in fact 'already begun', however this opinion has not been upheld by the UN.