It's likely the US nuke capability would be the last thing to go in any decline.

You can lose all of the toy soldiers and submarines and planes, but if you still have more nukes than anyone else, you're a playa - even if half of them won't work on demand.

I think a buy out of some sort is more likely. The Chinese are relatively smart, unlike the US political class, which is almost terminally stupid, so the Chinese are unlikely to start a war they can't win within taking heavy damage.

So I wouldn't be surprised to see conquest by osmosis, with China buying up the remains of the US corporations and introducing Chinese working practices to them, possibly with a figurehead or puppet president - in much the same way that Bush is a Saudi puppet, only rather more so.