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.
The decision to declare war was made by Adolf Hitler apparently offhand, almost without consultation. It has been referred to as Hitler's "most puzzling" decision of World War II.
Later that day, the U.S. declared war on Germany, with Germany's action having eliminated any remaining meaningful domestic isolationist opposition to the U.S. joining the European war.