 As clashes increased, casualties rose on both sides.After a victory at Korsun over the Polish army sent against them, the rebel Cossacks controlled a large territory that included Korsun, Pereiaslav, Kaniv as well as other cities, with Pereiaslav becoming their main base.In response to their successes, a large Polish army led by Stanisław Koniecpolski was sent to confront the Cossacks.