How do war-prone regions stabilize? This article tests realist, liberal, and constructivist hypotheses against the paradigmatic case of regional peacemaking, Western Europe after World War II. It concludes that none of the three theories perfectly explains the process of Franco-German reconciliation after 75 years of bitter conflict. Instead, the transition to stability occurred in two stages. The first stage, the transition, was driven by realist factors, principally the existence of a common Soviet threat and active American hegemonic participation. In the second stage, however, stability and cooperation were entrenched by liberal mechanisms, notably the institution of democratic political regimes and cooperative international institutions. I thus inductively arrive at the novel theoretical position that realist mechanisms may initially be required to compel regional rivals to put aside their differences, at least temporarily, and cooperate on the grounds of realpolitik, but liberal mechanisms are ultimately useful to sustain and deepen regional peace.