Except that it's a bit more complicated than that. Russia has nukes and is under attack from Ukraine, and while in the past they sabre-rattled that they would use tactical nukes if there was ever any incursion, they haven't done so because they know that would cause the whole world to retaliate.

Then there's nuclear defenses - if a country would have an effective anti-ICBM system (like Star Wars or whatever), it would make a nuclear counterstrike ineffective and end Mutually Assured Destruction. On paper anyway, in practice there are no perfect anti-ICBM systems, and they're effectively cluster bombs so in theory after the initial launch they can break up into half a dozen "dumb" nukes. Good luck hitting those.