The authors test Kantian and realist theories of interstate conflict using data extending over more than a century, treating those theories as complementary rather than competing. As the classical liberals believed, democracy, economic interdependence, and international organizations have strong and statistically significant effects on reducing the probability that states will be involved in militarized disputes. Moreover, the benefits are not limited to the cold war era. Some realist influences, notably distance and power predominance, also reduce the likelihood of interstate conflict. The character of the international system, too, affects the probability of dyadic disputes. The consequences of having a strong hegemonic power vary, but high levels of democracy and interdependence in the international system reduce the probability of conflict for all dyads, not just for those that are democratic or dependent on trade.