The empirical finding that democracies do not fight each other has long suggested that regime type influences international behavior. In the research reported here, we demonstrate that the democratic peace is only the most visible manifestation of the impact of domestic political structure on international relations. We consider the complete range of cooperative and conflictual interaction of each state in the international system with every dyadic partner in its politically relevant international environment between 1953 and 1978. We demonstrate a strong and consistent relationship between regime type and foreign policy behavior, both at the monadic and the dyadic level. States with more democratic characteristics tend to behave more cooperatively and less conflictually in the international system; these tendencies intensify when democratic states interact with other democracies. Meanwhile, non-democracies tend to treat democracies with more conflictual and less cooperative behavior. The influence of the internal institutional structures of states on their external behavior extends well beyond decisions to go to war. The empirical findings reported herein suggest that future research will benefit from increased attention to the relationships among state structure, leaders' incentives, foreign policy behavior, and international outcomes.