<P> On 11 December 1941, four days after the Japanese attack on Pearl Harbor and the United States declaration of war against the Japanese Empire, Nazi Germany declared war against the United States, in response to what was claimed to be a series of provocations by the United States government when the US was still officially neutral during World War II . The decision to declare war was made by Adolf Hitler, apparently offhand, almost without consultation . Later that day, the United States declared war on Germany . </P> <P> The course of relations between Germany and the United States had deteriorated since the beginning of World War II, inevitably so given the increasing cooperation between the United States and the United Kingdom . The Destroyers for Bases Agreement, Lend - Lease, the Atlantic Charter, the hand - over of military control of Iceland from the United Kingdom to the United States, the extension of the Pan-American Security Zone, and many other results of the special relationship which had developed between the two countries had put a strain on relations between the US, still technically a neutral country, and Nazi Germany . US destroyers escorting American supply vessels bound for the UK were already engaged in a de facto war with German U-Boats . Roosevelt's desire to help the UK, despite the objections of the influential US isolationist lobby, and legal impediments imposed by Congress which prevented direct involvement in the war, brought the US to push hard against the traditional boundaries of neutrality . </P>

When did us declare war on germany ww2