This chapter explores how the economic and political difficulties of the interwar period and the immediate post-war era forced politicians, economists, and institutions to re-think the European economic and monetary system. Triffin was a key figure in that dynamic and quick to spot the fundamental dilemma of international economic relations. The failure of the Bretton Woods system to restore multilateralism order led Triffin to consider that the regional approach was the appropriate level to achieve a stable international economic order. This vision gave birth to the European Payments Union in 1950 whose success produced the conditions to deal with monetary integration in depth in Europe.