My opinion on the whole matter, for what its worth.
America was a major contributor during WW2. That's what I believe. Would WW2 have been won without the US's help? Yes, however, it could have POSSIBLY taken longer to do it, and at the expense of more civilian and military casualties.
Would the US had even been involved in the war if Japan hadn't bombed Pearl Harbor?? Maybe, maybe not. But I think, and this is not fact, that the only reason the US got involved in Europe in the first place is because of Germany's alliance with Japan.
And if the US DIDN'T get involved in WW2, then the United States may have never been able to get out of the Great Depression this country was going through in the first place.
Now, with that being said, do you think if the US was around in the 11th century the Crusades may have ended up differently??