It was a major military operation of the Polish Army thanks to new alliance with the forces Military_operation of the Ukrainian People 's Republic under the exiled Ukrainian leader Symon Petliura .

In the Russian part of Ukraine Symon Petliura tried to defend Defending and strengthen Cause_change_of_strength the Ukrainian People 's Republic but as the Bolsheviks began Process_start to win the Russian Civil War , they started Process_start to advance Cause_to_make_progress westward towards the disputed Ukrainian territories , causing Causation Petliura 's forces to retreat Escaping to Podolia .