As horrible as nuclear weapons are, they have kept the world relatively stable because no country under sane leadership wants to invite the destruction a nuclear war would invite. Of course, the operating term here is sane. There have been times in the past that quite insane people have found themselves in positions of power, what happens then? Or worse, what happens if there is a miscalculation and a nuclear armed country drives another nuclear armed country to do the unthinkable? The other thing to fear is an accident. No man made system is perfect and an accident could be catastrophic… in the vein of the movie or book Fail Safe or the movie Dr. Strangelove.