Despite the enormous impact that war and the threat of war have had on human well-being, utilitarians have had surprisingly little to say about when, if ever, we may fight wars. Discussion of this question has been dominated by realism, pacifism and just war theory. This article takes some preliminary steps toward remedying this situation. I begin by spelling out what I call the Utilitarian War Principle (UWP). After presenting some considerations in its favour and answering some possible objections to it, I compare UWP with pacifism and with the principles of jus ad bellum found in the work of contemporary just war theorists. I argue that adherents of UWP should treat those principles as secondary moral principles, which, although subordinate to UWP, can and should guide its application and which, in turn, should be refined and revised with this goal in mind.