Wikipedia definition
1. Mongol invasion of Rus'The Mongol invasion of Rus was resumed on 21 December 1237 marking the resumption of the Mongol invasion of Europe, during which the Mongols attacked the medieval powers of Poland, Kiev, Hungary, and miscellaneous tribes of less organized peoples. The European invasion, heralded by the Battle of the Kalka River was fought between the scout forces of just the two Mongolian generals Subutai and Jebe, whose reconnaissance unit met in battle with the combined force of several Rus' princes.
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