Zelensky's move onto Russian territory with Ukrainian troops may prove the step to bring the Russians into negotiation.

Someone gnawing at your back--that's what the Ukrainians are doing to the Russians.

The strategy may work.  Nothing may work, if the Russians can't feel uncomfortable from the invasion of their space by another country's troops.

Go to it, Ukraine.  Some one hit-upon country has got to teach the Russians to leave other countries alone!  For, being independent is a precious status for a country to possess. 

My opinion.