What brought America into both world wars was the attack on American interests. Until then America was quite happy to play isolationism. But Japan had other ideas.
Britain could have pulled out of the ww2 but it seen its involvement as a duty to its allies. It is really a daft point about what europe would be like compared to America if Europe was united like the US, America would be in the mire.
As it is America complains about the way europe does business especially the EU, how would it be if Europe was united like the US, simple America would be the small fish as they would not be able to dominate europe as they try to do.
Please do not be naive to think that any country goes to war without the opinion of gaining or self interest. Why are questions raised about the oil and minerals in Afghanistan. Every thing has its price and rewards America made money in the last war.