If Putin uses a tactical nuke, the decision would fall into the hands of the US on whether to start a nuclear war. You need to think that through from an unbiased POV.If Putin were to use a tactical nuke he'd be playing a game that has no winners, with the odds of him being it's biggest loser. There are no scenarios where Putin comes away scoring some victory, none. Even china would abandon Russia. The only question left to ask is, how big can Putin lose, not whether he will.
Every time the question is proposed here on this board (and elsewhere) the complete answer is left hanging unanswered.
That could be because nobody has the stomach for facing the question.
The MSM has been guilty of the same thing. Just consider the impact on the American aucience if the question was answered completely to it's conclusion.
There is a very big effort being made to keep the impact of MAD out of the conversation. Even when it's becoming a tossup on whether Russia will use a tactical nuclear weapon.
The possibility of either the US or Israel using a tactical nuclear weapon on Iran has been thoroughly discussed in the past, without invoking the MAD theory.