sentences:  Although part of them was serving in the Polish army as Registered Cossacks, the Zaporozhian Cossacks remained fiercely independent and staged several rebellions against the Poles.In 1648, the peasants of what is now Ukraine joined the Cossacks in rebellion during the Khmelnytsky Uprising, because of the social and religious oppression they suffered under Polish rule.Initially, Cossacks were allied with Crimean Tatars, which had helped them to throw off Polish rule.