How International Law Works presents a theory of international law, how it operates, and why it works. Though appeals to international law have grown ever more central to international disputes and international relations, there is no well-developed, comprehensive theory of how international law shapes policy outcomes. Filling a conspicuous gap in the literature on international law, Andrew T. Guzman builds a coherent theory from the ground up and applies it to the foundations of the international legal system. Using tools from across the social sciences Guzman deploys a rational choice methodology to explain how a legal system can succeed in the absence of coercive enforcement. He demonstrates how even rational and selfish states are motivated by concerns about reciprocal non-compliance, retaliation, and reputation to comply with their international legal commitments.