The thing with Russia, as others have said before, is they have the tech and manpower to threaten the U.S. when very few other countries do. In all-out battle with most other countries in the world, the U.S. armed forces would greatly outnumber the enemy's. Other than that, most other countries would have to rely on a war of attrition type strategy, which could still be horrible, but it doesn't quite have the same punch as a full-on war that would kill millions or even destroy the entire world. So, you can either have a full-on war (or the threat of one) with Russia or a similarly large country, or you have to have some sort of sneaky guerrilla special forces kind of war which would be much less dramatic if it might spark a war with say...Barbados.
EDIT: Oh, and gaming gets enough flak in the U.S. as it is...imagine what those people would say if you were playing a game killing U.S. soldiers. We'd never hear the end of it. I'm all for morally challenging games, but it might be smarter to wait a while until games become somewhat more accepted as an art form and such before we start calling for moral ambiguity.