The story is really weird. North Korea manages to take over South Korea, China, Japan, and other countries, before invading the USA. If they somehow invented a super powerful technology before doing so (Nanobots! Super mech walkers! Deflector shields!), then it would make more sense.. but the info so far suggests they just got.... really angry. Anger is power!
Well, so long as the in-game story isn't stupid I can get over a somewhat stupid premise. It's plausible enough to not totally insult your intelligence, and honestly, the only people we can really fight are from the Middle East, and not only has that gotten boring but they have zero chance of a full-scale US invasion.
The idea of a modern war on home turf is an interesting one to play - has a certain impact and gives level designers so many interesting opportunities. But it's just not very plausible right now without bending reality a decent amount.