As a result of Hitler's invasion of Poland.
the U.S. declared war on Germany.
the U.S. declared war on all three Axis powers.
Germany declared war on Japan.
the U.S. decided to expand its military.
The U.S. wouldn't fully declare war until December 7, 1941, the day of the attack on Pearl Harbour. Additionally, the Japanese were staunch allies of the Germans during World War II.
