not only that, but the U.S has to be very careful where it targets since it has neighbors nearby. Fallout would be huge and would poison everyone in the region. The chinese could lose the entire population of the U.S and STILL have more people than America. They can also live in harsher conditions, no electricity, no running water, heck this is something many chinese had to go through just 2 decades ago. America has NEVER had to go through harsh times. It would be Katrina times 100, and with armed militia roaming around. There is no win win, just lose lose. The economy of the U.S crashing would pretty much put the entire world on pause. America has WAY more to lose than China (obviously).
This isn't a case of, we launch and go unharmed, its a case of america launches and everyone gets fukked up, and america loses it's superpower status overnight, while bringing in Russia retaliation and pretty much fukking up the whole planet.
Here's what one warhead can do.
http://3ammagazine.com/magazine/issue_4/articles/nuclear_bomb_effects.html
The U.S would back down it's aggressive stance if China had it's version of Bay of Pigs. Heck they back down from N.K. NK be launching missiles over japanese bases and we do nothing.
While that's true, I don't really think that is relevant. Total annihilation wouldn't happen...