Wikipedia definition
1. Mongol invasion of Volga BulgariaThe Mongol invasion of Volga Bulgaria lasted from 1223 to 1236. The Bulgar State, centered in lower Volga and Kama, was the center of the fur trade in Eurasia throughout most of its history. Before the Mongol conquest, Russians of Novgorod and Vladimir repeatedly looted and attacked the area, thereby weakening the Bulgar state's economy and military power. The latter ambushed the Mongols in the later 1223 or in 1224.
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