When I was at school, all the text books had the reason the US joined the war was because of Pearl Harbour. 

"JOINT RESOLUTION Declaring that a state of war exists between the Imperial Government of Japan and the Government and the people of the United States and making provisions to prosecute the same.

Whereas the Imperial Government of Japan has committed unprovoked acts of war against the Government and the people of the United States of America:"

That last phrase is usually taken as a reference to Pearl Harbour.

When the US declared war of Japan, Germany and the other Axis powers declared war on the US - as the US knew they would.