America gets invaded, what a great idea... but in fact, one nation could not do this. You need an alliance of nations to do this, probably a non-nuclear missile attacks on America also. If this is a modern campaign, it wouldn't be Russia who invades, but if it was still during the Cold War, I could see it, but I don't think Gorbachov (how ever you spell his name) would invade the U.S. if he had the chance... but hey, maybe he would. You could make it Vietnam era U.S. for a new flare, oh say 1975. I think the U.S. not in jungle warfare in 75 didn't wear the camo helmet they did in Vietnam, it was green... I think. It would make since that the USSR, seeing that America lost the Vietnam War, think they can beat the U.S. now.
And at the end of the campaign, the USSR takes over the U.S. and it would take the typical gamer by surprise.