After the defeat of the Khwarezmian Empire in 1220, Genghis Khan gathered his forces in Persia and Armenia to return to the Mongolian steppes. Under the suggestion of Subutai, the Mongol army was split into two forces. Genghis Khan led the main army on a raid through Afghanistan and northern India towards Mongolia, while another 20,000 (two tumen) contingent marched through the Caucasus and into Russia under generals Jebe and Subutai. They pushed deep into Armenia and Azerbaijan. The Mongols destroyed the kingdom of Georgia, sacked the Genoese trade-fortress of Caffa in Crimea and overwintered near the Black Sea. Heading home, Subutai's forces attacked the allied forces of the Cuman–Kipchaks and the poorly coordinated 80,000 Kievan Rus' troops led by Mstislav the Bold of Halych and Mstislav III of Kiev who went out to stop the Mongols' actions in the area. Subutai sent emissaries to the Slavic princes calling for a separate peace, but the emissaries were executed. At the Battle of Kalka River in 1223, Subutai's forces defeated the larger Kievan force. They also may have fought against the neighboring Volga Bulgars. There is no historical record except a short account by the Arab historian Ibn al-Athir, writing in Mosul some 1100 miles away from the event. Various historical secondary sources - Morgan, Chambers, Grousset - state that the Mongols actually defeated the Bulgars, Chambers even going so far as to say that the Bulgars had made up stories to tell the (recently crushed) Russians that they had beaten the Mongols and driven them from their territory. The Russian princes then sued for peace. Subutai agreed but was in no mood to pardon the princes. As was customary in Mongol society for nobility, the Russian princes were given a bloodless death. Subutai had a large wooden platform constructed on which he ate his meals along with his other generals. Six Russian princes, including Mstislav III of Kiev, were put under this platform and crushed to death.