What rem141 said. The much-derided concept of MAD, or Mutually Assured Destruction, is in fact the guarantee that there will never be a nuclear war between the major powers. Starting one would be not just national but global suicide, and all the major players know that.
That doesn't rule out some piss-ant terrorist group firing a nuke - or a radical country like Iran nuking Israel - but the major powers have more sense.
