Prior to the invasion, war with the Khwarazmian Empire and the usurpation of power by the Naiman prince Kuchlug had weakened the Qara Khitai.

[2][1] The invasion was precipitated when Kuchlug besieged the Karluk city of Almaliq,[3] which was a vassal of the Mongol Empire and whose ruler, Ozar, was married to a daughter of Jochi.

[7] At that point, Subutai turned southwest, destroying the Merkits and protecting Jebe's flank against any sudden attacks from Khwarazm.

[9][10] Jebe relieved Almaliq, then moved south of Lake Balkash into the lands of the Qara Khitai, where he besieged the capital of Balasagun.

When Jebe's army arrived at Kashgar in 1217, the populace revolted and turned on Kuchlug, forcing him to flee for his life.