Mongol invasion of Kievan Rus'Kiev, Ukraine
The Mongol Empire invaded and conquered Kievan Rus' in the 13th century, destroying numerous southern cities, including the biggest ones Kiev (50,000 inhabitants) and Chernihiv (30,000 inhabitants), with the only major cities escaping destruction being Novgorod and Pskov located on 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. 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 1242. 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 separate nations: modern-day Russia, Ukraine and Belarus.