South America is a region which is free from nuclear weapons. However, this was not an inevitable development from the relationships among its countries. Indeed, regional rivalries between Brazil and Argentina, with military implications for both countries, lasted a long time. After wwii these countries took part in the race to obtain nuclear technologies and nuclear ambitions were part of the game. In the mid 1980s, the end of military dictatorships and the successful establishing of democratic institutions put an end to the race. Thus regional and national interests in addition to the establishment of democracies in Latin America have been responsible for the building of trust between the two countries. Meaningful international initiatives are once again needed in the framework of worldwide cooperation. This cooperation is better developed when democratic regimes are in place. ; Fil: Freire Jr., Olival. Universidade Federal da Bahia; Brasil ; Fil: Hurtado de Mendoza, Diego Fabian. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de San Martín. Escuela de Humanidades. Centro de Estudios de Historia de la Ciencia y de la Técnica "José Babini"; Argentina ; Fil: de Castro Moreira, Ildeu. Universidade Federal do Rio de Janeiro; Brasil ; Fil: de Souza Barros, Fernando. Universidade Federal do Rio de Janeiro; Brasil