The Mongol conquest of Khwarezmia took place between 1219 and 1221, as troops of the Mongol Empire under Genghis Khan invaded the lands of the Khwarazmian Empire in Central Asia. The campaign, which followed the annexation of the Qara Khitai khanate, saw widespread devastation, including numerous war crimes, and marked the completion of the Mongol conquest of Central Asia.
Both belligerents, although large, had been formed recently: the Khwarazmian dynasty had expanded from their homeland to replace the Seljuk Empire in the late 1100s and early 1200s; near-simultaneously, Genghis Khan had unified the Mongolic peoples and conquered the Western Xia dynasty. Although relations were initially cordial, Genghis was angered by a series of diplomatic provocations. When a senior Mongol diplomat was executed by Khwarazmshah Muhammed II, the Khan mobilised his forces, estimated to be between 90,000 and 200,000 men, and invaded. The Shah's forces were widely dispersed and probably outnumbered — realising his disadvantage, he decided to garrison his cities individually to bog the Mongols down. However, through excellent organisation and planning, they were able to isolate and conquer the Transoxianan cities of Bukhara, Samarkand, and Gurganj.
Genghis and his youngest son Tolui then laid waste to Khorasan, destroying Herat, Nishapur, and Merv, three of the largest cities in the world. Meanwhile, Muhammed II was forced into flight by the Mongol generals Subutai and Jebe; unable to reach any bastions of support, he died destitute on an island in the Caspian Sea. His son and heir Jalal-al Din managed to mobilise substantial forces, defeating a Mongol general at the Battle of Parwan; he was however crushed by Genghis himself at the Battle of the Indus a few months later.