Errrr...you're the delusional freakshow around here. Why would the US want to invade China? You don't think the US could gain military bases on other countries bordering Russia or China? Such as Kazakhstan, for example? The US has military presence in Japan and South Korea and those are already on China's doorstep. Any military action the US would take against China would be from the air and both SK and Japan are well within range of US air forces, not to mention carrier battle groups.
Again, what makes you think the US is eager to invade China? China needs the US to survive financially, as it is China's largest market. NK doesn't matter to them at all and are more of a pain than anything. At worst, Skoorb's suggestion would probably happen -- China would want to be involved in any military operation in a "peacekeeping" role so they'd have a final say as to what happens with NK. The US has no intention of fighting China -- if that were America's plan, we would have done it in the 50s.
Oh, REALLY? You just said a few weeks ago that you were pretty sure that the US sank it but now, you "don't know." How interesting.
The US backed off the Yellow Sea for diplomatic reasons, not for fear of China. Additionally, regarding China having military drills with NK -- I can guarantee that the US definitely wouldn't be threatening nuclear war unlike your pals in NK. At any rate, the US military is decades ahead of anything NK has and China is probably 20 to 25 years behind as well.