In 1580, a group of Yermak Timofeyevich's Cossacks initiated the Russian conquest of Siberia, pushing eastwards on behalf of the Tsardom of Russia. They attacked the Vogul (Mansi) and Ostyak (Khanty) peoples in Yugra, and had also captured a tax collector of Kuchum Khan, the king of the large Tatar Khanate of Sibir, the most powerful force in the western Siberia region on the eastern side of the Ural Mountains. At the time the Voguls and Ostyaks were subjects of Kuchum, and in response the Tatars carried out retaliation attacks against the Cossacks.