Because I wouldn't. If I were Ukraine I'd just keep on killing Russians till either Russia imploded (again) or they left every square millimeter of Ukraine.
...me neither. Besides all this would be for Russia would be a timeout while they regrouped, rearmed and came back. The only way I see is to beat em so bad they run back under their rock and refuse to come out.
Liberals are like a "Slinky". Totally useless, but somehow ya can't help but smile when you see one tumble down a flight of stairs!