This article explores how Argentina and Chile put aside a century-long rivalry to form a dynamic regional partnership in the years after 1984. Their experience suggests that interstate behavior is more complex than many theories admit. Cooperation increased during and after the Cold War, with severe and moderate debt burdens, between economic liberalizers and statists, and under authoritarian and democratic regimes. This study uses institutional analysis to argue that executives were the indispensable actors who combined institutionally focused incentives and the ability to forge cooperative agreements. Previous attempts between Argentina and Chile, as well as elsewhere on the continent, failed when weak executives in one or both countries could not sustain cooperation over domestic opposition. Two crucial points are Alfonsín's and Pinochet's foundation-building agreements in 1984–85 and Menem's and Aylwin's deepening institutionalization of the relationship in 1990–91.