MAD. I don't think anyone in the US or China is going to nuke anyone else because of survival instincts. There must be chain-of-command and multi-party custody controls in the militaries of all nuclear powers to prevent unauthorized release. In such stable cases, they become worse than White Elephants: can't get rid of them because of the liabilities of inviting nuclear and conventional wars, and can't keep them because they're expensive, inherently dangerous, and tempting for crazy people to use.

Economic interdependence also creates another layer of protection from either side going too far because of the economic warfare potential before nuclear or conventional wars would be considered.