Whereas the literature on the democratic peace tends to treat the phenomenon as a causal law, we follow Immanuel Kant in interpreting it as a macro-historical process that expanded from a small number of democracies to about 50% of all states. In order to account for this development, we introduce an agent-based model that combines a natural-selection logic with an adaptive mechanism of regime change. The latter is implemented as an empirically calibrated, contextual rule that prompts democratization as an S-shaped function of the democratic share of a state's immediate neighborhood. A similar transition rule governs regime change in the opposite direction. The computational results show that regime change and collective security are necessary to produce realistic trajectories of democratization at the systemic level.