The connection between domestic politics and international cooperation, specifically the relationship between regime type and alliance behavior, is examined to test two central hypotheses: democracies are more likely to ally with each other, and states of any similar regime type are more likely to ally with each other. These hypotheses emerge from three theories: constructivism, economic interdependence, and credible commitments. The authors use a data set of all pairs of states from 1816 to 1992. Results show that states with similar regime type are more likely to ally with each other after 1945, although two democracies are not more likely to ally than two autocracies during this period, and distance, learning, threat, and common culture affect alliance behavior, but trade does not. Results indicate sharp limits to the connection between democracy and international cooperation.