This dissertation examines the parallel historical development of nuclear technology and diplomacy in Argentina and Brazil between the end of World War II and 1995, when the neighbors accepted and adhered to bilateral and international weapons nonproliferation measures, then led broad economic integration efforts on the continent. Brazil’s and Argentina’s pursuit of autonomous nuclear energy capabilities has vexed political scientists, who have produced some excellent scholarship on a historical process of building and refining technology, diplomacy, and law; nonetheless, these developments defy most models to explain them. As a work of history, this dissertation recasts this process as the interplay of two mutually constitutive pairs. Nuclear technology and diplomacy, linked since before the bombing of Hiroshima and Nagasaki, played a fundamental role in shaping Argentina and Brazil, connected by geography and competition for nearly 500 years. Both nations began this period by trading newly valuable nuclear minerals to the hemispheric superpower, the United States, but developmentalist governments in the South American neighbor countries invested quickly and heavily in beginning the human and physical infrastructures for nuclear energy. Only with a fearless and forceful early start, political leaders and scientists believed, could the gifts of the Atomic Age lead to economic and social benefits for the people of Argentina and Brazil, vault each country out of middle-power dependency and above the geopolitical vicissitudes of the Cold War. In this way, the two nations would complete the elusive process of technological autonomy from multinational corporations and North Atlantic technology transfer partners, a possibility that their diplomats defended vociferously in the drafting of the Treaty of Tlatelolco (1967) and outright rejection of the United Nations Non-Proliferation Treaty (1968). Political leaders, military generals, and scientists in both nations continued to believe in this transformative power of nuclear energy, and made expensive bets on a future where it would be integral to continued industrial development. The goal to complete the nuclear fuel cycle in Brazil and Argentina exemplified and intensified a complex, competitive bilateral relationship for influence and power on the continent, particularly from the mid-1960s to the mid-1980s, when both nations were under military government. A serious and continuous effort to ensure cooperation on peaceful use of nuclear energy began in diplomatic and high political circles nearly a decade before the return of electoral democracy to either country, while efforts to master the sensitive processes of uranium enrichment, heavy water production, and spent fuel reprocessing continued unabated. But by 1995, both nations had ceased early-stage weapons development programs, accepted full safeguards and international verification of all nuclear activities, and transformed the “imported magic” of nuclear energy technology into their own. How this all happened, and why, is the story of the parallel power play at the heart of this dissertation.