The ubiquity and importance of war have made analyses of its causes a central concern of scholars for over two millennia. Many of the fundamental questions about the causes of war were raised by Thucydides in the fifth century B.C., but the vast amount of work on the topic since that time has produced ongoing debates instead of generally accepted answers. Studies of war can be divided into three broad categories (reviews of the literature using similar frameworks are provided by Waltz 1959; Bueno de Mesquita 1980; and Levy 1989). The first type takes the system as whole as the unit of analysis and focuses on how characteristics of the interstate system affect the frequency of war. States are the unit of analysis in the second type, which explores the relationships among the political, economic, and cultural features of particular states and the propensity of states to initiate wars. The...