Battle of Kalka River (31st May, 1223 N.S.) was the first military engagement between the Mongol armies of Genghis Khan and the East Slavic warriors. It was fought on the bank of the Kalka River, somewhere between present-day Donetsk and Mariupol.

Early in 1223, the Mongol armies under Subedei Bahadur and Jebei Noyon 'the Arrow' approached the steppe occupied by the Kipchak nomads. Khan Kotian of the Kipchaks escaped to the court of his son-in-law, Mstislav of Halych, and asked him for help, saying: "Today they will slaughter us, tomorrow they will come for you". Several other princes, including Mstislav the Bold, joined their forces with Kipchak allies in Kiev and sailed down the Dnieper. There was no unity in the Slavic camp, as the princes were political rivals and each hoped to obtain a supreme command over the army.

The Mongols sent several embassies to the Slavic princes offering peace. They asked Mstislav and his army to let them have their way with the Kipchaks and promised not to raid into Slavic lands. Mstislav however was persuaded in his own victory and had the ambassadors killed. The princes then espied an avantgarde of the Mongols and pursued it for 8 days until they reached the fateful Kalka River.

Mstislav of Kievs forces, who had chosen to cross the river, were attacked and besieged in their camp by the main body of the Mongol horde. The Kipchak allies retreated in disarray, but Mstislav the Bold stood firm. The camp was assaulted for three days, and finally taken by cheating. There was no mercy to anyone. Six princes were taken prisoner, and only a tenth of the original army escaped (led by Mstislav the Bold) to return to Kiev. The imprisoned princes were stretched out under the wooden boards and slowly suffocated while Mongols feasted upon the boards during their victory banquet.

The Battle of the Kalka River is commonly viewed as a catastrophe in a disintegrating Kievan Rus. The Mongol commanders, however, were not inclined to conquer Rus at that time. Genghis Khan viewed their mission as a mere reconnaissance in force to prepare a better attack in the future. The Mongols returned under the leadership of Batu Khan more than a decade later, in 1239. For the rest see the Mongol invasion of Russia.