Not entering the war right away was, in retrospect, a truly brilliant move - although I know that if I were an allied government at the time, I'd be super pissed at America. It allowed you to boost your economy and convert your industrial infrastructure to war-time needs. It also gave you the time needed to build a massive military machine, to train your troops and to let Hitler get bogged down by being overly ambitious.
I've said it a thousand times before: America was and is at it's best when it waits until absolutely necessary to strike... and when it does strike, it uses overwhelming force to brutalize the enemy. This is the America people know and love... it was your adventures in empire building that, as always, inspire the vitriol and hatred that comes along with such power.