I'm pretty sure if China invaded they wouldn't do it in the open. Think about it, they have a massive ocean to cross, they would be sitting ducks. I would think they would load troops up into ocean tankers that should be carrying oil, commercial goods, things like that. They aren't a stupid people.

Also, if war did break out, I doubt it would be as straightforward as us vs them, I'm sure the rest of the world would join in.