To understand the momentous transformation in Argentine-Brazilian relations from rivalry to Mercosur, scholars need to analyze negative cases, when rapprochement was attempted unsuccessfully. This article examines the failed 1972 summit between Presidents Alejandro Agustín Lanusse and Emílio Garrastazú Médici, which is poorly explained by existing theories of international relations and overlooked or misinterpreted in many regional histories. I argue, based on research in the Argentine Foreign Ministry Archives, newly declassified US government documents, and a reexamination of published primary sources, that rapprochement failed in 1972 primarily because bureaucratic interests in the armed forces and foreign ministries of both states depended on the continuation of rivalry. Organizational politics, not popular nationalism or presidential diplomatic errors, best explains the persistence of Argentine-Brazilian conflict in the early 1970s. Successful cooperation between rivals therefore may require not only agreement between national leaders but also the support of the state apparatus on both sides.