Adolf Hitler declared war on America, on December 11, 1941.
On December 8, 1941, one day after the attack on Pearl Harbor, the United States declared war on Japan. This prompted Germany to declare war on the United States, which, in turn, led to the United States declaring war on Germany on December 11, 1941.
Why did Germany declare war on the United States?
Wilson cited Germany’s violation of its pledge to suspend unrestricted submarine warfare in the North Atlantic and the Mediterranean, as well as its attempts to entice Mexico into an alliance against the United States, as his reasons for declaring war.