This article explores the evolutionary and endogenous relationship between democracy and war at the system level. Building on Kant, the authors argue that the rules and norms of behavior within and between democracies become more prevalent in international relations as the number of democracies in the system increases. The authors use Kalman filter analysis, which allows for the parameters in the models to vary over time. The results support the propositions that democratization tends to follow war, that democratization decreases the systemic amount of war, and that the substantive and pacific impact of democracy on war increases over time.