weakness will always invite aggression
I figure it this way; if Russian tanks stop at the east bank of the Dnieper River, fine. After all, that part of Ukraine is heavily Russian.
If the tanks are in Lviv (right on the Polish/Ukrainian border), there could be a big problem. Poland, Slovakia, Hungary and Romania - all of whom share a border with Ukraine - are all NATO member states.
Poland is getting nervous right now. And Lithuania and Latvia are set to invoke Article IV of the NATO Treaty, and I suspect Poland is going to add to that chorus real soon.