Until recently, rulers routinely treated the punishment of affronts as a legitimate reason to make war. Today, warmaking to punish misbehaving princes may seem no better than warmaking to grab land, tribute, or glory. Under the Charter of the United Nations and customary international law, only self-defense counts as a legitimate reason to go to war. To be sure, individual political and military leaders can be punished for war crimes, including the crime of aggression. The International Criminal Court exists for just that purpose. But punishing leaders in a court of law is not the same as using warfare itself as the instrument of punishment. We may think that punishment by the sword, like wars of conquest, represents a lesser stage of civilization than we aspire to. This transformation in thinking about just cause raises two important questions: First, how did we get from there to here—from widespread acceptance of punishment as a just cause for war to widespread rejection of it? Second, and more important, is the question of whether punishment of wrongdoing might actually be a just cause for war despite the modern narrowing of just cause to self-defense.