This article advances several arguments about scientific and diplomatic collaboration on peaceful nuclear energy technologies between Argentina and Brazil. First, although 1972–1980 is generally viewed as an acrimonious period in the neighbors’ bilateral relationship, nuclear energy officials in these South American countries made significant and unexpected progress toward technical cooperation, motivated by a shared geopolitical position. Furthermore, technological differences between the two nuclear energy programs, and experts’ ideas on their potential implications for agreement between key officials, opened potential avenues for diplomatic cooperation. Finally, this article illuminates a surprising quality of diplomatic behavior of the military dictatorships in Brazil and Argentina. Against the backdrop of the Cold War, Argentina and Brazil shared nearly identical stances on the unfettered right to pursue advanced nuclear technology. In combination with increased technical cooperation, this ideological alignment supported a landmark bilateral presidential summit in 1980 on nuclear energy collaboration.