I'm hardly an expert on the subject, but from what I see, China's economic advantage doesn't hold up, ultimately.
From my understanding, America is in debt from China because we sell them the raw material, and they're the one's who process the stuff, turn it into a real product, and sell it back for a higher price. Cutting that link would be bad for both parties- China would have its main supply of stuff taken away, and we'd be losing our 'factories'.
That said, America has proven in WW2 that, when unified and the situation calls for it, can step the hell up. This isn't as true nowadays, since all the factories we converted for the sake of the war are now in China and America runs largely on service-based jobs, but the optimist in me thinks that we could pull it off again. Question remains if we'd have the time, though. I wouldn't believe that either China or America would be stupid enough to actually fire a Nuke, but there were dozens of times in the Cold War when malfunctions made each side THINK they were under fire, and it was only the presence of a calm mind that prevented disaster.
Russia is sort of a Non-factor. History has shown that invasions of Russia end badly for the invaders, but as far as attacks from them go, it wouldn't be noteworthy, from what I know. Its easy to forget that 30 years isn't actually that long in the great scheme of things- they were the USSR not too long ago, and are still recovering from the transition.
That's just my take...
thanks