Cossacks are a group of predominantly East Slavic people who originally were members of democratic, semi-military communities in Ukraine and Southern Russia. They inhabited sparsely populated areas and islands in the lower Dnieper and Don basins, and played an important role in the historical development of both Ukraine and Russia. The origins of the first Cossacks are disputed. Traditional historiography dates the emergence of Cossacks to the 14th to 15th centuries. Towards the end of the 15th century, the Ukrainian Cossacks formed the Zaporozhian Sich centered on the fortified Dnipro islands. Initially a vassal of Poland-Lithuania, the increasing social and religious pressure from the Commonwealth caused them to proclaim an independent Cossack Hetmanate, initiated by a rebellion under Bohdan Khmelnytsky in the mid-17th century. Afterwards, the Treaty of Pereyaslav brought most of the Ukrainian Cossack state under Russian control for the next 300 years.
Contributions by Altenmann, 18.104.22.168, and Kuban kazak.