I don't think they could - I think they could kill many Americans, but in the end, we would easily be the one left standing.
Hey, I'm not saying you couldn't wipe out ther rest of the world too, just that you are not invincible! There's no shame in that!
Just because we could, it doesn't mean we would have. We *could* have won the war, but killed innocent people in the process. We made a tactical mistake by going in at all, but that doesn't mean we couldn't have wiped them all out with the push of a button.
How about war movies where the good guys don't always win...I've noticed that regardless of anything, Americans always come out on top...
I would probably say Platoon is a good example - they take some heavy losses in the final scene. Mind you, they still win and live to fight another day. I think you are right - all the wars they lost, they just tend to make films about the battles that they won.
What about the Great Escape? That had some yanks in it!