MAD Deterrence and Mad Leaders

During all the fateful years of the Cold War, the world survived due to the rational  reasoning that since both parties to the ideological conflict were in possession of enough nuclear weapons to destroy the other, many times over, and since each of them had the capacity of a second strike should it be taken by surprise and attacked with its guard down, none of them would be irrational enough to launch a nuclear strike and none of them would take the responsibility for destroying the world, erasing  mankind