Why do some Americans act they won WWI and WWII on their own...?

Because they get their history from the films. Don't forget, according to the film they got the enigma machine. Seriously though, Most don't. And don't forget, we have our very own fools on this side of the Atlantic. Just as an aside to them you could mention that they only came in for the glorious last bit of both wars, when there wasn't much left to do apart from mopping up. ;-)))