The campaign was conducted from April to June 1920. It was a major military umdertaking of the Polish Army, which operated in alliance with the forces of the Ukrainian People's Republic led by Petliura. Their adversary was Soviet Russia, which claimed most of the territories for the Ukrainian SSR afterwards. Initially successful for the Polish and Ukrainian armies which captured Kiev (Kyiv) on 7 May 1920, the campaign's progress was dramatically reversed, chiefly due to the actions of the cavalry of Semyon Budyonny.