iirc the US had very little to do with fighting the Nazis in general (if anything)
The US really just kind of brushed aside the beleaguered German armies in France at the end of the war and pushed them out of the France, the low countries and Western Germany. They did fight in Africa and Italy as well though, and although arguably the Brits could have won without them there's no denying they sped things up, which was crucial considering the fact that the Reds were killing supermen at an alarming rate. If the US hadn't entered the European theatre there is a good chance the Netherlands, Belgium, Denmark, Austria and maybe even Italy would have had communist governments (or governments-in-exile) and West Germany wouldn't have existed at all.
Some people will claim the Germans were a hard fight in France, but there is no denying they were ready to be knocked over by anyone willing to engage them on even terms for more than a couple hours.
Anyways the US did save most of the Pacific from being controlled by the Japanese, although they did a pretty crummy job on the mainland ("should we help the Nationalists fight the Communists? naw what harm can a couple of red Chinese do?" "oh dang, we forgot about Korea"). Now we have the PRC and North Korea, and we also have the Vietnamese police action (not a war guys, right?

) do remember their amazing legacy by!
Does anyone remember the Wind and the Lion starring Brian Keith as Teddy Roosevelt and Sean Connery as the Raisuli? That is how the US should handle it's foreign policy. Have a bunch of Marines in dress uniform run up to their enemies, shoot them in the face while the rest of the world watches, and then leave. Good ol' Teddy Roosevelt. He would have ended the war on terror in no time flat. He'd also have Osama stuffed and put on display in the Smithsonian when he was finished with him.
edit: it sounds like I'm ragging on the US but international politics is pretty hard so they've done a pretty good job, all things considered. I do think that the US has had a major impact on the foreign policies of every other nation on the Earth since WW2, though, just because they exist. It's a hard thing not to take into consideration!