While a substantial body of theory suggests that democracies should behave peacefully toward all states (monadically), most empirical evidence indicates they are only pacific in their relations with fellow democracies (dyadically). A new theoretical synthesis suggests that the missing link between democratic constraints and pacific monadic behavior is leaders' perceptions of, and responses to, these constraints. Research on political leadership indicates that, contrary to conventional wisdom, leaders respond in systematically different ways to domestic constraints: “constraint respecters” internalize constraints in their environments, while “constraint challengers” view such constraints as obstacles to be surmounted. An analysis of 154 foreign policy crises provides strong support for this contingent monadic thesis: democracies led by constraint respecters stand out as extraordinarily pacific in their crisis responses, while democracies led by constraint challengers and autocracies led by both types of leaders are demonstrably more aggressive.