After being badly defeated by Russians under Dmitri Donskoi first in 1378 at the Battle of the Vozha River and then at the Battle of Kulikovo (1380), Mamai began to assemble a large force to punish Dmitri. But he was defeated by Tokhtamysh on the banks of the Kalka River and was either executed by Tokhtamysh or assassinated by the Genoese after fleeing to Caffa. The death of Mamai paved the way for the next khan to reunify the Golden Horde.