A preventive war is a preemptive war in which the requirement of an imminent threat is relaxed. The present paper examines the justifiability of preventive war. First, it lays out the prevailing doctrine of just war and explains why preventive war does not fit comfortably within it. It then discusses the justification of the prevailing doctrine, first by examining Michael Walzer’s rights-based justification for it, which it rejects, then by offering a broadly consequentialist justification, very close to the thinking of the U.N.’s founders. The most important point emerging from these arguments is that the real justification for the prevailing doctrine lies in the importance of a no-first-use-of-force rule for war prevention, not in the importance of protecting state sovereignty. The paper then turns to the question whether a general doctrine permitting preventive war to forestall immature threats is morally defensible, and answers no, arguing that giving a green light to preventive war would make wars too frequent and too routine. However, a more restricted form of the doctrine, which permits preventive war against serious threats posed by rogue states, is sound under certain conditions.
