From this excellent post:
The reason MAD – Mutually Assured Destruction – worked was that at heart the Soviets didn’t want to kill off all of their population just as the US didn’t want to kill off all of its population. When push came to shove, nobody wanted to destroy the world and wash it in blood.
Most modern mailings of this story end with something like, “This was back in the days when there was honour in being a warrior. They proudly wore uniforms, and they didn’t hide behind women and children, nor did they plant bombs amidst innocent crowds. How times have changed..” And this difference in value systems, this willingness to kill innocent bystanders, and the eagerness to wash the world in blood – this is the main difference we face today.