The Mongol conquest of Khorasan took place in 1220-21, during the Mongol conquest of the Khwarazmian Empire. As the Khwarazmian Empire disintegrated after the capture of the large cities of Samarkand and Bukhara by the Mongol Empire, Shah Muhammad II fled westwards in the hope of gathering an army. Genghis Khan ordered two of his foremost generals, Subutai and Jebe, to follow the Shah and prevent any such Khwarazmian resurgence; meanwhile, he sent his youngest son Tolui south to subjugate any resistance.