mutual assured destruction?

What's wrong with this picture?

us, them
World powers in ideological, economic, and territorial conflict, each possessing massive nuclear armament. Not intended as an endorsement of actual us-them thinking, which tends to suck.
L
Launch those puppies.
~L
Don't.

An assumption: A full-scale nuclear assault is going to have overwhelmingly negative outcomes for anybody actually trying to live on the surface of the Earth, sufficient to outweigh the gains from reducing a rival power's territory to so much radioactive kipple even should they be unable (or unwilling) to retaliate. Since a first strike is certain to at least make things suck for everyone, and likely to result in an equally destructive response, no rational actor has any incentive to fire first.

This is not exactly an illustration of MAD, which asserts that retaliation is certain (this says nothing directly about the probability of a decision, it only suggests the results), but it is not unrelated. I find myself less than satisfied with the whole thing, and resolve some day to understand why on a more rigorous level.

As usual, the Wikipedia article on MAD is informative. See also: Game theory, and balance of terror, which is closer to saying the same thing as my little graph.