 Cossacks were warriors organised into military communities, resembling pirates and pioneers of the New World.In 1648, the peasants of Ukraine joined the Zaporozhian Cossacks in rebellion against Poland-Lithuania during the Khmelnytsky Uprising in reaction to the social and religious oppression they had been suffering under Polish rule.In 1654, the Ukrainian leader, Bohdan Khmelnytsky, offered to place Ukraine under the protection of the Russian Tsar, Aleksey I. Aleksey's acceptance of this offer led to another Russo-Polish War.