The Otrar Catastrophe was a siege that took place between December 1219 and February 1220 during the Mongol invasion of the Khwarazmian Empire at Otrar, a large trading city on the Syr Darya river. Inalchuq, the city's governor, had seized the goods of a Mongol trade caravan the previous year; after more provocations from Inalchuq's liege and ruler of the Khwarazmian Empire, Shah Muhammad II, Genghis Khan launched a full-scale invasion of the empire.
The city had been extensively garrisoned and fortified, and the Mongol troops found it difficult to breach the battlements. Progress was slowly made, and by February Genghis felt confident enough to detach part of his army and head southwards towards Transoxiana. His sons Chagatai and Ogedei were left behind to continue the siege. Qaracha, the leading general of the city, deserted in February 1220 and the inner citadel fell soon afterwards. Inalchuq was captured alive, and was executed. Some sources relate that he was executed by having molten metal poured into his orifices; this story, symbolising his greed in seizing the caravans, is almost certainly apocryphal.