Throughout the Second World War British and American companies competed to gain the contract for the electrification of the central Brazilian railway. The British Foreign Office used this case to establish a broader principle with the US government that the conditions brought about by war would not be used by one country to gain commercial advantage at the expense of the other. While the US government supported this principle in theory, this article argues that they failed to adhere to it in practice. US actions in this case shed new light on the country’s economic diplomacy with Britain during World War II.