What is the purpose of the international law on armed conflict, and why would opponents bent on destroying each other’s capabilities commit to and obey rules designed to limit their choice of targets, weapons and tactics? Traditionally, answers to this question have been offered on the one hand by moralists who regard the law as being inspired by morality, and on the other by realists who explain this branch of law on the basis of reciprocity. Neither side's answers withstand close scrutiny. In this Article we develop an alternative explanation which is based on the principal-agent model of domestic governance. We pry open the black box of "the state," and examine the complex interaction between the civilian and military apparatuses seething beneath the veil of sovereignty. Our point of departure is that military conflicts raise significant intra-state conflicts of interest that result from the delegation of authority to engage in combat: between civil society and elected officials, between elected officials and military commanders, and within the military chain of command. We submit that the most effective way to reduce domestic agency costs prevalent in war is by relying on external resources to monitor and discipline the agents. Even though it may be costly, and reciprocity is not assured, principals who worry that agency slack may harm them or their nations' interests are likely to prefer that warfare be regulated by international norms. The Article expounds the theory and uses it to explain the evolution of the law and its specific doctrines, and outlines the normative implications of this new understanding of the purpose of the law. Ultimately, our analysis suggests that, as a practical matter, international law enhances the ability of states to amass huge armies, because it lowers the costs of controlling them. Therefore, although at times compliance with the law may prove costly in the short run, in the long run it strengthens the state against its enemies.