The problem is the idea that it is rationnal for a leader to consider that if one use a nuke, therefore another should use it as retaliation.
The very idea of using a Nuke in the first place, no matter if it is for defense or attack should be seen as as vile and evil as raping and killing a thousand kids.
I don't want my country to annihilate an entire civilisation whose governement used a nuke, even if said nuke was directed directly at me and even if 90% of the opposite faction are fascists.
The best defense against a nuke is not to use another nuke.. It is to stop the cycle of violence right at the moment of first launch.
To take the hit...
And let history destroy the ones who were too foolish to control themselves.