A nuclear holocaust is exactly what Japan tried to avoid by surrendering to the Allies at the end of World War II, and nobody in their right mind wants a repeat of Hiroshima or Nagasaki. China won't nuke the US for fear of reprisals by the US, nor vice versa.
Earlier someone said something about the economy being completely global now, and that none of the major countries of the world has anything to gain from a third World War. For the moment, this seems to be the case, but the world's economy has taken a beating over the last few years, and countries may wish to make contingencies in case the economy continues to decline.
There may be a "self-sufficiency race" of sorts, as countries try to outdo each other on reducing their dependence on imports. The country that becomes self-sufficient fastest will be in the best position to launch the first strike, as it will have the least to lose from attacking another country.