A Mongol reconnaissance force under Subutai lured the combined armies of several Rus' principalities and their Cuman allies into a trap along the Kalka River in present-day Ukraine. The Rus' coalition, riven by rivalry and poor coordination, suffered catastrophic losses, with most of its princes killed. The defeat exposed the Rus' principalities' vulnerability decades before the full Mongol conquest arrived.
By 1223 the Mongols under Genghis Khan had already crushed the Khwarazmian Empire in Central Asia, and a scouting expedition of perhaps 20,000 horsemen under generals Subutai and Jebe pushed westward into the Pontic steppe, defeating the Cumans along the way.
The Cuman khan Köten appealed to his Rus' relatives for help, and princes from Kiev, Chernigov, and Galicia assembled a large coalition army, reportedly outnumbering the Mongols significantly. But Rus' politics were fractious; the princes distrusted one another and refused unified command, some even fighting the Mongols separately without informing allies.
Subutai staged a nine-day tactical retreat to draw the coalition into overextension, then turned and annihilated the pursuing force at the Kalka River. Six Rus' princes died, including Mstislav of Chernigov, and only a fraction of the army escaped. The Mongols withdrew afterward, this being merely reconnaissance, but the battle's lesson went unheeded. Thirteen years later Batu Khan returned with the full Mongol war machine, and the divided Rus' principalities fell one by one.