Pereyaslav Agreement, Pereyaslav also spelled PerejasŁaw,  (Jan. 18 [Jan. 8, Old Style], 1654), act undertaken by the rada (council) of the Cossack army in Ukraine to submit Ukraine to Russian rule, and the acceptance of this act by emissaries of the Russian tsar Alexis; the agreement precipitated a war between Poland and Russia (1654–67).
The hetman of the Zaporozhian Cossacks, Bohdan Khmelnytsky, had been leading a revolt against Polish rule in Ukraine since 1648. In 1651, in the face of a growing threat from Poland and forsaken by his Tatar allies, Khmelnytsky asked the tsar to incorporate Ukraine as an autonomous ...http://global.britannica.com/EBchecked/topic/451403/Pereyaslav-Agreement

http://northhistories.blogspot.ru/2014/09/pereyaslav-treaty-of-1654-agreement.html

http://en.wikipedia.org/wiki/Treaty_of_Pereyaslav

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