Since the mid-16th century, portions of what is today Ukraine found themselves under Polish rule, and social and religious antagonisms grew in strength. A set of uprisings turned into the 1648 Ukrainian War of Liberation led by the Cossack hero Bohdan Khmel'nytskyi (ca. 1594–1657). By 1654, the Orthodox Ukrainians faced a choice among being subject to Catholic Poland, being allied with Muslim Turkey, or seeking the protection of their co-religionists in Moscow. At Pereiaslav, the assembly (rada) swore allegiance to the Russian tsar. Russia’s concurrent war with Poland ended in 1667 with the Treaty of Andrusovo, which divided Ukraine along the Dniepr (in Ukrainian, Dnipro) River, with Kiev and the left bank going to Russia. The clergy of Ukraine, educated along Catholic lines, played a major role in introducing western European ideas into Muscovy.