1. Assuming cooler heads prevail, and UN unanimously sides with the USA, China will be given a chance for a limited surrender (no occupation) after turning over ALL personnel in the command chain leading to the attack, starting with the entire political leadership, for a war crimes trial before an international tribunal. If China to surrender refuse then one of the follow scenarios results.
If China will not surrender, and the UN support remains, then the next step is a rigidly enforced total blockade and embargo of China to provoke an economic collapse and unconditional surrender. If the UN will not support, then go to 2.
2. If cooler heads do not prevail, or unanimous UN support can not be had, then the USA will launch a nuclear counterstrike. Most likely this will be using Trident missiles launched in a suppressed trajectory to take out China's ICBM capacity. If successful this would be followed by additional strikes at military facilities (most of which are in or adjacent to large population centers resulting in significant non-military casualties) to weaken the Chinese military to the point that the US Navy can enforce a blockade without UN assistance or permission and starve the central government into submission.
3. If, at any point, China launches its ICBMs then the USA will salvo its Chinese targeted ICBMs in response. Depending on various factors the Chinese will kill a dozen cities and 10%-25% of the US population, probably much less if the USA has gotten ABM systems (Navy SM-3) deployed and in position . The USA will destroy all Chinese cities and 40%-70% of the population, mostly ethnic Chinese. With the central government gone, or at the very least crippled, the relatively untouched ethnic border areas (western China and Mongolia) will rise up and revolt. Russia, after assessing the results of the US strike, will launch it's own nuclear strikes to finish the job. This will be followed by invasions from Russia, Taiwan, Japan, and possibly Vietnam, in a race to grab the best pieces of the corpse. The surviving ethnic Chinese will be a minority in whatever arrangement results.
� 1998 -