I wouldn't claim to be a geopolitical mastermind but my understanding is that Russian control/influence over Ukraine is geopolitically vital for the existence of the Russian state.

Without it Russia has nothing between them and the west. Nothing defendable that is. Just a straight open plain right into the Russian heartland.

So knowing that i would assume Russia would literally fight for their life to maintain keeping western influence in Ukraine to the smallest possible. Because doing so is fighting for their life