The US has a major security advantage in terms of mutually assured destruction in that its home to many people from around the world. There are like tens of thousands of Americans living in all of Russia or China. Meanwhile, there are millions of both Russian and Chinese Americans in the U.S.. This provides safety. Good luck finding morale in your military after you've just told them to nuke millions of their own people.
mutually assured destruction will not leave you any functioning military to have morale after nuking US. And in this case we will test the nuclear winter theory. Hopefully we can avoid that.
