Cossacks (Ukrainian: Козаки́, Kozaky, Russian: Казаки́, Kazaki, Polish: Kozacy) are a group of predominantly East Slavic people who originally were members of democratic, semi-military communities in Ukraine and Southern Russia inhabiting sparsely populated areas and islands in the lower Dnieper and Don basins and who played an important role in the historical development of those nations.

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 around 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.

Together they began a systematic conquest and colonisation of lands in order to secure the borders on the Volga, the whole of Siberia, the Yaik and the Terek Rivers.

The Don Cossack Host, which had been established by the 16th century, allied itself with the Tsardom of Russia. Together they began a systematic conquest and colonisation of lands in order to secure the borders on the Volga, the whole of Siberia, the Yaik and the Terek Rivers. By the 18th century, Cossack hosts in the Russian Empire served as buffer zones on her borders. However, the expansionist ambitions of the empire relied on ensuring the loyalty of Cossacks, which caused tension with their traditional independent lifestyle. In the 17th and 18th centuries this resulted in rebellions led by Stenka Razin, Kondraty Bulavin and Yemelyan Pugachev. In extreme cases, whole Hosts could be dissolved, as was the fate of the Zaporozhian Sich in 1775. By the end of the 18th century, Cossacks were transformed into a special social estate (Sosloviye); they served as border guards on national and internal ethnic borders (as was in the case in the Caucasus War) and regularly supplied men to conflicts such as the numerous Russo-Turkish Wars. In return, they enjoyed vast social autonomy. This caused them to form a stereotypical portrayal of 19th century Russian Empire abroad and her government domestically.

After the dissolution of the Soviet Union, the Cossack lifestyle and its ideas have made a return in Russia.

During the Russian Civil War, Cossack regions became centres for the Anti-Bolshevik White movement, a portion of whom would form the White emigration. The Don and Kuban Cossacks even formed short-lived independent states in their respective territories. With the victory of the Red Army, the Cossack lands were subjected to famine, and suffered extensive repressions. After the dissolution of the Soviet Union, the Cossack lifestyle and its ideas have made a return in Russia. There are Cossack organizations in Kazakhstan, Ukraine and other countries.

(Source: Wikipedia)