Pathbreaking and controversial, Darwin and International Relations offers the first comprehensive analysis of international affairs of state through the lens of evolutionary theory. Bradley A. Thayer provides a new method for investigating and explaining human and state behavior while generating insights into the origins of human and animal warfare, ethnic conflict, and the influence of disease on international relations. Using ethnological and statistical studies of warfare among tribal societies, Thayer argues that humans wage war for reasons predicted by evolutionary theory -- to gain and p