I wasn't thinking MAD specifically, although I'm aware of at least one example where Russian early warning picked up an incoming object that appeared to them to be a missile, and they didn't respond, which seems very un MAD. I was thinking of the proxy wars, Vietnam, Korea, etc. I'm not even sure the threat of nuclear attack was used. I suppose you could say that's part of MAD (not attacking allies), if that were the case, wouldn't the same reasoning extend to not fighting them in the first place, in the same way there were never any conventional wars between Russia and the US?

So agreed it is mainly about self preservation, but I would say it goes a little further than that.