Democratic peace studies have traditionally identified Kantian “republicanism” with procedural democracy and largely ignored liberalism and constitutionalism, which are even more fundamental for Kant's reasoning behind the liberal peace. A closer look into his major political works reveals that peaceful relations are expected from states with the protection of individual freedoms (liberalism), the rule of law and legal equality (constitutionalism), and representative government (democracy). Only when all three constitutive elements are jointly considered can we uncover the multifaceted nature of Kant's approach to the domestic sources of international peace. In this way, we not only find that monadic and dyadic expectations are consistent with Kant's theory, but also that both normative and interest-based explanations for international peace can equally draw on Kant as their theoretical precursor. We further demonstrate that it is plausible to infer that the Kantian legacy is related to civil peace as well. The propositions we derive from our theoretical reexamination of the Kantian legacy are strongly supported in our quantitative empirical test. Moreover, constitutional liberalism, rather than democracy, shows to be both more central for Kant's theory and empirically more robustly related to international as well as domestic peace.