Excerpts from Books and Wikipedia
"The primary intention had been to conquer the Siberian Khanate, allowing Russian expansion into the region, which was under the leadership of Khan Kuchum. . . . However, Kuchum regrouped his forces. In 1584, he killed Ermak and regained control of his ruined capital." [Furtado: 1001 Days] "Russians' fur-trade interests fueled their desire to expand east into Siberia. The Tatar khanate of Kazan was established as the best entryway into Siberia. In 1552, Ivan the Terrible's modernized army toppled the khanate. After the takeover of Kazan, the tsar looked to the powerful and affluent Stroganov merchant family to spearhead the eastward expansion. In the late 1570s, the Stroganovs recruited Cossack fighters to invade Asia on behalf of the tsar. These Cossacks elected Yermak as the leader of their armed forces, and in 1582 Yermak set out with an army of 840 to attack the Khanate of Sibir." [Wikipedia]