This essay makes three claims about preventive war, which is demarcated from preemptive war and is part of a broader class of ?anticipatory? wars. Anticipatory wars, but especially preventive war, are ?hard cases? for traditional Just War theory; other puzzles for this tradition include nuclear deterrence, humanitarian intervention, and provability a priori of the success of Tit-for-Tat. First, and despite strong assertions to the contrary, it is far from clear that preventive war is absolutely prohibited in traditional Just War Theory, (...) and it is also dubious that it is in all cases ?clearly illegal?. Second, the morality of both preemptive and preventive wars is shown to turn on epistemological considerations: on what degree and kind of justification the primary metaphysical facts of threat can be reasonably believed. Third, an argument is made that whatever epistemic threshold is held to be necessary, some preventive wars will exceed it, and that this is more likely with advancing technologies of information acquisition. Finally, the common argument that allowing all nations to follow policies of preventive war would result in more wars than barring such policies is shown to be mistaken by simulations in game theory. Suggestions are made about the derivation of traditional Just War criteria from more basic moral principles, and about their subtle failures as jointly necessary conditions for the morality of war. (shrink)