That was never the Soviet's game, they planned to win if it came down to it, and note all the mentions of 2nd World shelters in this discussion for part of that plan. I've also read a translated version of their basic civil defense book, it was good.
MAD was insanity cooked up by Robert Strange McNamara and company of Vietnam infamy in the early-mid-60s. The Soviets never bought into it, and thought, correctly, that a plan to burn alive children in their homes was irredeemably evil, and a sign the US was very evil. And at this level it was and still is.
Although emphasize could, it was hardly certain. (And if this strikes you as crazy, well, remember that since a little before 1920 they explicitly were planning on taking over the world, and got uncomfortably close to achieving that goal.)
This was one of the reasons SDI, even on paper/in the lab, was so terribly effective. Against an even partly effective ABM system a counterforce first strike becomes impossible, since you can't choose which warheads or boosters get intercepted.
Counterforce is an attack on your opponent's strategic forces, aimed at preventing them from retaliating. That's one of the reasons we had to build the expensive "Triad" of ICBMs, SLBMs, and manned bombers. Reliably taking all three out is very difficult.
MAD was insanity cooked up by Robert Strange McNamara and company of Vietnam infamy in the early-mid-60s. The Soviets never bought into it, and thought, correctly, that a plan to burn alive children in their homes was irredeemably evil, and a sign the US was very evil. And at this level it was and still is.