Joseph is exactly right, but we didn’t know that we could rely on that then, and we still don’t now. Crazies don’t care if they will be wiped out too.
I don’t know how I really feel about disarmament. It’s certain that no country will do so unilaterally. But even if all countries disarm, nuclear technology can be redeveloped pretty easily as long as the materials exist (and there’s no “peak-uranium” in sight, ever). I don’t think there can ever again be a world without nuclear power. So is it a good idea to get rid of weapons which developing countries might conceivably have tomorrow?