I'm of the firm belief that China would stomp the shit out of us in combat. I'm not saying we suck by any means, but let's face it - one country would be more than willing to nuke another to extinction... and we know what country that is. Furthermore, let's talk size of military; China itself boasts a 4:1 population advantage over us, with a whopping 7,024,000 military units (counting Active, Reserves and Paramilitary forces), whereas the U.S. has 3,385,400 units. You may bring up our superior weaponry (and let's face it we have awesome shit), but China isn't underpowered by any means; we must also remember that even a good chunk of our current forces overseas have crap armor and supplies (especially when compared to the private firms that were over there). If we can't even supply (or worse, won't) many our guys and gals with basic armaments, how the hell would we plan to supply them in an all out war with China?
Looks like we need to get cracking on more unmanned war machines, like the recently created heavy armored tank treaded gatling gun, or EATR (a robot that is capable of consuming biomass to keep itself running).