It's ok to kill as long as you're diong it as an american coalition soldier.
Simulated killing of a simulated soldier, first of all. And a lot of games allow you to kill civilians, police, and so forth, many rewarding you if you do. Why does it matter if the illusory people are American soldiers? I personally have no problem with "firing" a collection of pixels that looks like a gun at a collection of pixels that looks like a human, whether that human resembles a criminal, a Nazi soldier, an American solder, a guy on the street, or my grandmother. I despise the idea of killing any conscious being in reality, but I can separate that physical reality from the simulated reality of a game. (Usually. The Tangle Vines in Metroid Zero Mission really freak me out.) If you (general "you", here) cannot draw that line and are emotionally or morally effected by the deaths of simulated characters, then don't play the games. But yes, killing a fake anything
as part of a larger storyline or test of skill is morally acceptable to me.