This chapter evaluates international cooperation during the Cold War, particularly Anglo-Chilean cooperation in nuclear science and technology. It depicts negotiations between the Chilean government and a British firm, Fairey Engineering, as well as the involvement of a number of British government departments, resulting in an agreement for the construction of a British-made nuclear reactor in Chile. It highlights an engaged, imaginative, and risk-taking British position in Latin America, while reconstructing Chile’s early nuclear history, from 1955 to 1970. This episode is contextualised within the larger story of the establishment of the international nuclear science community, organised via the International Atomic Energy Agency, the formation of which opened new channels of international cooperation in the 1950s.