This chapter presents a third strategy of regional peacemaking — regional integration — and its application to Western Europe in the era following World War II. This strategy has produced high-level warm peace in the region. The conditions for its success are, however, quite demanding to establish and include both domestic factors — at least a minimal extent of state-to-nation balance and liberalization — and external ones — in particular, a benign great power environment. The external conditions are especially crucial in the early stages of the regional integration when the internal conditions are still evolving. As these conditions (liberalization and state-to-nation balance) reach a high level, however, the role of the external factors becomes less crucial for maintaining the integration and high-level warm peace. Thus, US hegemony, motivated by the Soviet threat common to both the United States and Europe, was a key to the emergence of a cold European peace in its early stages when state-to-nation problems were addressed and democracy was emerging, especially in Germany. Following the resolution of the state-to-nation issues and the consolidation of democracy, regional integration developed and US hegemony became less necessary to maintain the high-level warm West European peace.