The chain of events that led to the Mongol invasion started when the Shah of Khwarazm, Shah Muhammad II, broke a peace treaty that he had agreed with Genghis Khan. On the Shah's orders, the governor of the city of Otrar arrested and put to death all the members of a Mongol trade caravan; when the Khan, trying to avoid open war, sent three diplomats to the Shah at Urgench, one was beheaded and the others were publicly humiliated. Outraged by this affront, Genghis left the wars he was fighting in China, and prepared to attack Khwarazm
they say the mongols killed like 15million in that war alone
they say the mongols killed like 15million in that war alone