structural:summarization, stylistic:tone, structural:reformatting
Cossacks

Most Cossacks failed to enter the noble estate and continued their role as free soldiers. The lower rank Cossacks often resented their wealthier brethren and were responsible for frequent rebellions, particularly during the Ruin, a period of instability and civil war in the 17th century. These resentments were frequently exploited by Russia. The Zaporizhian Sich served as a refuge for Cossacks fleeing the Hetmanate as it had been prior to Khmelnytsky's uprising.