The US was "lending" warships to the UK, and American destroyers were dropping depth charges on German submarines in an effort to get supplies the Germans were trying to choke off to the UK. In the North Atlantic we'd already joined the war, if unofficially and not wholeheartedly. This is one of the reasons Hitler decided to honor his ally's request to make it official.

In the Pacific US forces were essentially blockading Japan's oil supplies in an effort to force the Japanese out of Manchuria. Japan had to go to war with the US or give up any imperial ambitions (and we knew that wasn't going to happen).