This article explores the evolutionary and endogenous relationship between democracy and war at the system level. Building on Kant, we argue that the rules and norms of behavior within and between democracies become more prevalent in international relations as a whole as the number of democracies in the system increase. The aggregation of the dyadic democratic peace implies that the degree of war in the international system may increase with democratization, but over a certain threshold further democratization reinforces the positive relationship between democracy and peace. Moreover, the relationship is endogenous since war is positively related to the spread of democracy in the international system. War increases the share of democracies in the system because democracies tend to win wars more frequently than non-democracies, and because states losing in war often experience regime changes. Juxtaposing these mechanisms, we argue that the relationship between democracy and war is evolutionary, as war increases democratization and democratization reduces war. We test these propositions using Kalman filter analysis, which allows for the parameters in our models to vary over time. Our results support the propositions – democratization tends to follow war, democratization decreases the systemic amount of war, and the substantive and pacific impact of democracy on war increases over time.