This paper provides an economics-based interpretation of the standard finding in the literature that democracies rarely fight each other. A general theory of conflict between two countries is presented and empirical analysis applies this theory to the question of why democracies rarely fight each other. The results show that the fundamental factor in causing bilateral cooperation is trade. Countries seek to protect wealth gained through international trade, therefore trading partners are less combative than nontrading nations. Democratic dyads trade more than nondemocratic dyads, and thus exhibit less conflict and more cooperation.