Russia’s nuclear doctrine that nuclear weapons will only be used if the very existence of the state itself is threatened. That rules out all of these calls for baby nukes, tactical nukes, etc. As I understand it, this means there has to be a threat of the state itself actually falling. On that basis, Stalin could have used nuclear weapons if he had them when the Germans were at the gates of Moscow but not before. Russia also has a “no first use” policy on nuclear weapons.
The only countries that have used tactical nuclear weapons since WW2 are two of the most evil nations on Earth, the US and Israel.