[mad=dog]

Nuclear weapons have a tendency to underkill. Overkill is accomplished by a tremendous amount of these weapons, not the weapon itself. During the cold war era peace was maintained through parity and MAD (Mutually Assured Destruction). Since the amount of nuclear weapons needed for complete destruction is so immense such an action had to be deliberate and each block was able to do it to one another. Hence total destruction of the other meant total destruction of yourself. The equivalence of the entire chain of command simultaniously putting a gun to their head and pull the trigger.
A single nuclear weapon is still a weapon of awesome (British awesome, not US awesome) power. Not something that should be treated lightly.
As for WWIII it's not happening anytime soon. The previous World Wars have all been about resources. While oil is a resource western democracies are eager to control I don't see that control contested by a powerblock of equal size. Actually that is the only thing preventing WWIII from breaking out as the world is still largely colonial with the first world trying to control the third world and its abundant raw materials for our industries.