Afterwards, the Treaty of Pereyaslav with Russia, secured Ukraine within the growing state. Initially a vassal of Poland-Lithuania, the increasing social and religious pressure from the Commonwealth in the mid 17th century, forced them to proclaim an independent Cossack Hetmanate, which began a massive rebellion under Hetman Bohdan Khmelnytsky. The Dnieper Cossacks of Ukraine formed the Zaporozhian Sich centered around the fortified Dnieper islands. By the start of the 16th century, they swelled into large militant states. These were fuelled by many migrants who left the adjacent northern states of Moscow and Lithuania due to social and religious pressures. In the 16th century, the Don Cossacks established another cossack host in the Don River basin. Towards the end of the 15th century, Zaporozhian Cossacks had established a Cossack host in the wild field of Ukraine around the Dnipro River. True origins are disputed, and many theories exist on their formation, but the Cossacks enter history as members of military communities in what is today Ukraine and Southern Russia, inhabiting sparsely populated areas and islands in the lower Dnieper basin. Ећablon:Cossacks Cossacks Russian: РљР°Р·Р°РєРёМЃ, Kazaki ( Ећablon:Lang-uaВ Ећablon:Lang-pl) are a group of predominantly East Slavic martial people presently numbering up to 7 million people. For other uses, see Cossack (disambiguation). Not to be confused with KazakhsВ or Cassock.

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