[jcranmer]

A war between the US and China is most likely to result in a stalemate. China may not have the capabilities to threaten continental US possessions to any degree, but it does have the ability to deny amphibious invasion. The coast of China would be an active war zone, with neither side able to move with impunity.

Beyond that, it's hard to say what would happen without knowing who would be involved in the way, which depends a lot on how the war was started. The main land routes into China are either via Russia (unlikely to permit staging of US troops), Kazakhstan (ditto), Kyrgyzstan via Tajikistan and Uzbekistan (aka the Silk Road, and dubious due to enough tensions between those countries), Burma (again dubious permission), Vietnam (which might prefer to try to remain neutral), and North Korea (which would only happen in the context of a North-South Korea war). The passes over the Himalayas from Pakistan, India, and Nepal are insufficient quality to maintain an armed front. Recall the difficulties of supplying Chinese forces in WWII.