I realize this is probably highly implausible, but here goes:
The scenario is that, at any point between 1970 and 1991, either the US or the USSR launch their missiles.
Is there any chance at all that the other power will not retaliate, on the basis of their leadership deciding that there is no point to a retaliatory strike? That after the war, it would be less impossible to rebuild if at least one major power is still around? Or not wanting to stain their country's name with pointless bloodshed when the writing is on the wall?
Or would the response to a missile launch be automatic and unstoppable once detected?