People have been looking at this for a long time and the general consensus is that it would be impossible to occupy the entire CONUS. China could take the the west coast and hold it. A reinforced Canada could go 20-50 miles across the border and maybe hold it. Same with Mexico if it had outside help. From the Rockies to the Appalachian would be impossible to occupy and hold. Without major support from Europe or Russia there is no way anyone is taking the east cost and the supply lines would be too weak and vulnerable from china. You have to factor in India and Russia to anything China would do. While they would all like control over the U.S. none of them want the other to have control over the U.S. Then you have to factor in all the weaponry owned and controlled by the U.S. Just the planes and ships we have mothballed in the various bone yards could really destabilize a large country including China. The amount of advance weaponry that is outdated but still immensely lethal just lying around with minimal protection is staggering. Because of this alone, if the U.S. government fails and is incapacitated for over a few months then the entire world is coming with us. The only real play to take over the U.S. would involve a slow disarmament of all bases and citizens. This does seem to be underway. However, this process will take 20-50 years. As this process become more obvious, it doesn't take much for some rogue group on a destroyer, sub, or land based iCBM to glass Tel Aviv or Jerusalem and take out a few key damns and stuff in China.