The idea is that if you promise to drop nuclear weapons on any country which attacks your capital, Russian tanks will not attack your capital. Whether this is a wise strategy is unclear, but countries like Pakistan[1] and Israel[2] have employed such a policy, and none of their unfriendly neighbors have yet managed to send tanks into Islamabad or Tel Aviv.

[1] https://en.wikipedia.org/wiki/Full_spectrum_deterrence

[2] https://en.wikipedia.org/wiki/Samson_Option

Nukes are arguably ONLY useful in defense (and deterring another from attacking). Using nuclear weapons against an opponent that also has nuclear weapons will very likely lead to a nuclear exchange and the decimation of both countries. It’s suicide, and so no one is going to do it unless they have nothing left to lose. A nation under attack, facing an existential threat, and about to be overrun, is in exactly that circumstance.

And yeah, maybe the defending countries leader isn’t willing to end it all, and maybe try collaboration or partisan resistance. But if you were the attacking country, would you be willing to make that bet? Or would you hesitate?

You use tactical nuclear weapons, as was planned by NATO in case of attack of Warsaw Pact tanks through Fulda Gap

https://en.wikipedia.org/wiki/Fulda_Gap

In reality this escalates very fast into all out nuclear war. Which is why you don't see wars between two nuclear armed countries. There is no winner in a nuclear war.