In this chapter I will reassess the ad bellum principle of proportionality, especially in light of the reconceptualized Just War theory I have defended in previous chapters, as well as by reference to contemporary international law. After setting out the ad bellum principle of proportionality, I will return to the question of whether the doctrine of self-defense can be used to justify a preventive or preemptive war. Within the Just War tradition, there are actually two principles of proportionality that are not necessarily related to each other. There is both a jus ad bellum and a jus in bello principle of proportionality. The jus ad bellum principle of proportionality, the focus of this chapter, primarily concerns limitations on when initiating or waging wars might be unjustified because the gains aimed at in the war do not outweigh the likely losses in that war. The jus in bello principle of proportionality, the subject of other writings of mine, concerns tactical considerations during war, primarily whether a given tactic otherwise thought to be justified is rendered unjustified because it causes unnecessary or superfluous suffering. Both proportionality principles concern weighing, and both principles can render otherwise justified acts unjustified during wartime. But they are thought to be strictly unrelated to each other in that, at least according to traditional Just War theory as well as contemporary international law, no matter how disproportionate the tactics, the proportionality of initiating or waging war is not affected.