The contingent morality of war: establishing a diachronic model ofjus ad bellum

According to most accounts of just war theory, jus ad bellum is concerned with the morality of initiating war. This gives jus ad bellum a temporal dimension, making it a set of principles that are applied to judge belligerents’ actions at the outset of a war, but that cannot be revisited after a war begins. I challenge this synchronic conception of jus ad bellum by arguing that the considerations the principles of jus ad bellum are meant to judge can, and often do, change substantially over the course of wars. It is inappropriate to determine the ad bellum justice of a war solely based on how the principles of jus ad bellum are satisfied at a war’s outset. Because of the mercurial nature of war, jus ad bellum principles should be applied diachronically, as moral norms that can be used to guide or to judge belligerents even after a war has been initiated.