In this far-reaching exploration of the evolution of warfare in human history, Jack S. Levy and William R. Thompson provide insight into the perennial questions of why and how humans fight. Beginning with the origins of warfare among foraging groups, The Arc of War draws on a wealth of empirical data to enhance our understanding of how war began and how it has changed over time. The authors point to the complex interaction of political economy, political and military organization, military technology, and the threat environment-all of which create changing incentives for states and other actors. They conclude that those actors that adapt survive, and those that do not are eliminated. In modern times, warfare between major powers has become exceedingly costly and therefore quite rare, while lesser powers are too weak to fight sustained and decisive wars or to prevent internal rebellions. Conceptually innovative and historically sweeping, The Arc of War represents a significant contribution to the existing literature on warfare.