a lot is made of MAD, but it has to be remembered that both sides' own people (scientists, soldiers, generals) were largely and painfully aware that thermonuclear war would have likely been excessively disastrous, regardless of whether the other side could shoot back. This was already fairly clear after bombing Japan, to people who had the right information, but over the subsequent 20 years it became widely understood. While politicians saber-rattled, their own subordinate hierarchies often recoiled in horror. When even scientists start going "well, we probably shouldn't learn more about that", you know you're on the edge of existence.