Registered Cossacks
The Ostryanyn uprising was a 1638 Cossack uprising against the Polish–Lithuanian Commonwealth. It was sparked by an act of the Sejm (legislature) passed the same year that declared that non-Registered Cossacks were equal to ordinary peasants in their rights, and hence were subjected to enserfment. The uprising was initially led by Cossack Hetman Yakiv Ostryanyn () but was eventually crushed.