So nukes don't exist?
If nukes were such an obvious cheat code to victory, Moscow would have just nuked Kiev instead of struggling in embarrassment for 4 years.
The reason why Russia hasn't used nukes against Ukraine is simple: it would cause Russia to be isolated completely from China and even seen as a unhinged potential enemy and it would most likely be seen as an attack on nato and if nothing else would normalize nuke proliferation as now countries who don't have nukes sees that a new precedent.
No, but I think the threat of nuclear holocaust will make it so Russia won't move beyond their little toy excursions.
As far as I know, nuclear powers have never been invaded before.