Book excerpt: Why did some countries transition peacefully from communist rule to political freedom and market economies, while others did not? Why did the United States enjoy a brief moment as the sole remaining superpower, and then lose power and influence across the board? What are the prospects for China, the main challenger to American hegemony? In Complexity Science and World Affairs, Walter C. Clemens Jr. demonstrates how the basic concepts of complexity science can broaden and deepen the insights gained by other approaches to the study of world affairs. He argues that societal fitness—the ability of a social system to cope with complex challenges and opportunities—hinges heavily on the values and way of life of each society, and serves to explain why some societies gain and others lose. Applying theory to several rich case studies, including political developments across post–Soviet Eurasia and the United States, Clemens shows that complexity science offers a powerful set of tools for advancing the study of international relations, comparative government, and, more broadly, the social sciences.