I've often wondered about this (for context, I'm from the U.S.).
Now, I don't mean games where U.S. soldiers are enemies straight up. Modern Warfare 2, I went out of my way to kill as many of those traitors at the end of the game as I could. I even walked up and stabbed the two crewmen after the helicopter crash. The betrayal stung, but the fact that these were U.S. soldiers stung even more (yes, I am bias toward my country's troops and hold them to a higher standard).
And then there's Spec Ops: The Line. I felt bad about killing them for a few moments, but when it became clear that it was them or me, they just became enemies, regardless of country. Again, I didn't really see them as U.S. once I saw how far they had fallen either.
But a game where I'm straight up attacking U.S. troops? Like, say, playing as a German soldier gunning down U.S., or even Allied troops, during Normandy? Or playing as, shall we just go with Russia, soldier invading the U.S. and killing U.S. troops defending the country? I don't know. I think that would make me very uncomfortable.