The siege of Bukhara took place during the Mongol conquest of the Khwarazmian Empire, in March 1220. Genghis Khan, ruler of the Mongol Empire, had launched a multi-pronged assault on the Khwarazmian Empire, ruled by Shah Muhammad II. While the Shah planned to defend his major cities individually, the Mongols laid siege to the border town of Otrar, and struck further into Khwarazmia.