Attack on Pearl Harbor
Japan declared war on the United States and the British Empire the same day. Due to an error, the British and Americans did not know until the next day. The British government declared war on Japan immediately after being attacked. On December 8th, the United States Congress declared war on Japan. On December 11, Germany and Italy declared war on the United States. The U.S. which responded with a 1941 declaration of war upon Germany and a declaration of war upon Italy.