nobles as part of the Grand Duchy of Lithuania.
By 1569, the Union of Lublin formed the Polish–Lithuanian Commonwealth, and a significant part of Ukrainian territory was moved from Lithuanian rule to the Crown of the Kingdom of Poland, thus becoming Polish territory. Under the cultural and political pressure of Polonisation, many upper-class people of Polish Ruthenia (another term for the land of Rus) converted to Catholicism and became indistinguishable from the Polish nobility. Thus, the commoners, deprived of their native protectors among Rus nobility, turned for protection to the Cossacks, who remained fiercely Orthodox. The Cossacks tended to turn to violence against those they perceived as enemies, particularly the Polish state and its representatives.
In the mid-17th century, a Cossack military quasi-state, the Zaporozhian Host, was established by the Dnieper Cossacks and the Ruthenian peasants fleeing Polish serfdom. Poland had little real control of this land, yet they found the Cossacks to be a useful fighting force against the Turks and Tatars, and at times the two allied in military campaigns. However, the continued enserfment of peasantry by the Polish nobility, emphasized by the Commonwealth's fierce exploitation of the workforce, and most importantly, the suppression of the Orthodox Church pushed the allegiances of Cossacks away from Poland.
The Cossacks aspired to have representation in Polish Sejm, recognition of Orthodox traditions and the gradual expansion of the Cossack Registry. These were all vehemently rejected by the Polish nobility, who had power in the Sejm. The Cossacks eventually turned for protection to Orthodox Russia, a decision which would later lead towards the downfall of the Polish–Lithuanian state, and the preservation of the Orthodox Church and in Ukraine.
In 1648, Bohdan Khmelnytsky led the largest of the Cossack uprisings against the Commonwealth and the Polish king John II Casimir. Left-bank
