Preemptive war is a traditionally-accepted justification of war; is a subset of defensive war in which the attack it is responding to is an imminent attack (one in progress) rather than one that has completed.<p>What was controversial in the Bush Administration was an attempt to expand the concept of preemptive war to include what is also referred to as &quot;preventive war&quot;, where the attack is not &quot;imminent&quot; as that concept had been traditionally understood, but a more distant presumed eventuality. This had been traditionally (and generally remains, despite the efforts made by that administration) viewed as a subtype of aggressive war.<p>(&quot;Traditionally&quot;, here, means particularly in the post-WWII, under-the-UN-Charter, world in which states are viewed to operate under a regime of international law in which resort to force, except in immediate self-defense -- possibly collective -- or as directly authorized by the Security Council is forbidden. Though this generally follows other theories of war and its legitimate justification that make similar distinctions, such as Catholic Just War doctrine.)