Besides, in case of a global nuclear exchange only about one fourth to one third of the world's population will die (including all factors according to the most pessimistic scenarios). Some more will die out of hunger later, but generally - it wouldn't be all that disasterous as many people think. The humanity will live on. In fact, in the world of global superpowers there is no place for other players so many nations will get a second chance once the main players will be out of the game.

P.S. What I mean is that some ideas start to pop in the minds of high-ranked generals around the world. By 2020 US might be able to neutralize all Russian nuclear forces and I doubt there will be anything that would stop them from using nukes again.