This thesis examines relations between the United States and Great Britain in South America between 1939 and 1945. It does so in the broader context of the economic planning for the post-war world undertaken by the US and Britain during the Second World War. Traditional interpretations of Anglo-American post-war economic planning have tended to focus on a process whereby the Franklin D. Roosevelt administration advocated a multilateral system, based on equality of access to markets and raw materials. Doubting Britain’s ability to compete successfully in such a system, the British government baulked at the US proposal and clung to its autarkic structures constructed during the interwar years. This thesis argues that relations between the US and Britain in South America followed a different and more complex pattern. In this region it was in fact Britain that eventually took the lead in advocating multilateralism. This policy was adopted following a lengthy evaluation of British policy in Latin America, which concluded that multilateralism represented the surest means of protecting British interests in South America. The US, on the other hand, demonstrated exclusionary tendencies in its policy toward Latin America, which threatened the successful implementation of a global economic system based on multilateralism. In explaining this divergence from multilateralism in the Roosevelt administration’s post-war economic planning, this thesis pays particular attention to the influence of different factions, both within the administration and in the broader US political and business establishment. By exploring Anglo-American relations in this previously neglected region, this thesis contributes toward a greater understanding of the broader process of post-war economic planning that took place between the US and Britain during the Second World War.