 Complete the following phrase:  Russia continued its territorial growth through the 17th century, which was the age of Cossacks. 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. Alekseys acceptance of this offer led to another 