This chapter explores the diplomacy of Jefferson Caffery, United States ambassador to Brazil between 1937 and 1944. As an experienced Latin American specialist Caffery was able to exert extensive influence in Brazil. However, the outbreak of the Second World War meant that Caffery’s authority in the country was challenged on a number of fronts. These challenges came both from within the Franklin D. Roosevelt administration, and from internal political developments within Brazil. Exploring the ways in which Caffery sought to meet these challenges highlights some of the broader issues that arise when diplomacy is practiced during a time of war.