The Mongol Empire invaded and conquered Kievan Rus' in the mid-13th century, destroying numerous cities including the largest such as Kiev (50,000 inhabitants) and Chernigov (30,000 inhabitants), with the only major cities escaping destruction being Novgorod and Pskov, located in the north. The campaign was heralded by the Battle of the Kalka River in May 1223, which resulted in a Mongol victory over the forces of several Rus' principalities as well as the remnants of the Cumans under Köten. The Mongols retreated, having gathered their intelligence, which was the purpose of the reconnaissance-in-force. A full-scale invasion of Rus' by Batu Khan followed, from 1237 to 1241. The invasion was ended by the Mongol succession process upon the death of Ögedei Khan. All Rus' principalities were forced to submit to Mongol rule and became vassals of the Golden Horde, some of which lasted until 1480. The invasion, facilitated by the beginning of the breakup of Kievan Rus' in the 13th century, had profound ramifications for the history of Eastern Europe, including the division of the East Slavic people into three distinct separate nations: modern-day Belarusians, Russians, and Ukrainians, and the rise of the Grand Duchy of Moscow. Moscow started its independence struggle from the Mongols by the 14th century, ending the Mongol rule (the so-called "Mongol yoke") in 1480, and eventually growing into the Tsardom of Russia. Background As it was undergoing fragmentation, Kievan Rus' faced the unexpected invasion of a foreign foe coming from the mysterious regions of the Far East. "For our sins", writes the Rus' chronicler of the time, "unknown nations arrived. No one knew their origin or whence they came, or what religion they practiced. That is known only to God, and perhaps to wise men learned in books". The princes of Rus' first heard of the coming Mongol warriors from the nomadic Cumans. The historical accounts after the initial invasion call them by the name Tartars. They were previously known for pillaging settlers on the frontier, the nomads now preferred peaceful relations, warning their neighbors: "These terrible strangers have taken our country, and tomorrow they will take yours if you do not come and help us". In response to this call, Mstislav the Bold and Mstislav Romanovich the Old joined forces and set out eastward to meet the foe, only to be routed on April 1, 1223, at the Battle of the Kalka River. Although this