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 t
he system increases. The authors use Kalman filter analysis, which allows f
or 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.