In 1648, a number of ethnic groups within the Ukraine rebelled against the Roman Catholic King John II Casimir. Initiated by Cossacks the war of liberation soon attracted their fellow Orthodox Christians: Ukrainian peasants and Crimean Tatars. The rebels – commanded by Bohdan Khmelnytskyi, a Zaporozhian Cossack who gave his name to the uprising – managed to drive the Polish nobles, Catholic priests, and Jewish leaseholders from their land in the first few months of the insurgency – massacring those who did not flee. Meanwhile, the King was building an army to take back the Ukraine.
On 28th June 1651, the largest battle of the seventeenth century began when approximately 140,000 rebels engaged just over 50,000 Commonwealth soldiers under the command of the king at Berestechko in the western Ukraine. The battle lasted for three days by the end of which between forty- and seventy-thousand rebels lay dead (including women and children at their camp), while the Polish-Lithuanian army had lost less than one-thousand men. The rebels were forced to capitulate and signed the Treaty of Bila Tserkva on 28th September.
In spite of the defeat, Khmelnytskyi (aka Chmielnicki) had not given up hope of forcing the Commonwealth out of the Ukraine, but he realised that he needed allies to do so. Initially he approached the Ottoman Sultan who offered the rebels vassal status; however, the Ukrainians were not keen on a Muslim overlord. So it was that Khmelnytskyi signed the Treaty of Pereyaslav in 1654, making the Ukraine a vassal state of the co-religionist Russian Tsar, finally ending Polish-Lithuanian domination of the Ukraine.
Herman Rosenthal’s article on JewishEncyclopedia.com gives a Jewish perspective of the Cossacks’ Uprising.