Chinggis Khan's invasion of the Khwarazmian empire in 1219, provoked by the killing of a Mongol trade mission, destroyed the largest Islamic state of its day in two years. Bukhara and Samarkand fell in 1220, Urgench, Merv, Nishapur and Herat in 1221; where cities resisted, the Mongols expelled the population, separated out artisans for deportation, and killed the remainder. The Persian chroniclers give figures that dominate the event's reputation: Juvayni reports 1.3 million killed at Merv and Ibn al-Athir gives comparable totals elsewhere, with claims for Nishapur and Herat above a million. Modern specialists reject these. John Masson Smith's work on the logistics of Mongol siege warfare, and David Morgan's assessment of the Persian sources, argue that the cities concerned could not have contained such populations and that the numbers function as expressions of catastrophe rather than counts. That the campaign killed on a very large scale, wrecked the irrigation agriculture of Khurasan and Khwarazm, and permanently reduced several great cities is not disputed; the arithmetic is.
Chinggis Khan's armies destroyed the Khwarazmian empire, sacked Bukhara, Samarkand, Merv, Nishapur, Herat and Urgench, killed or deported their populations and drove captive levies ahead of their assaults
No defensible denominator exists. Estimates of the Khwarazmian empire's population range from under 5 million to over 10 million depending on how much of Iran, Khurasan and Transoxiana is counted and at what date, and the empire itself had existed in its full extent for barely a decade. The medieval chroniclers' city figures - Juvayni's 1.3 million dead at Merv, comparable numbers for Nishapur and Urgench - are rejected as arithmetically impossible by modern scholars, and no fiscal register survives to replace them.
The range reflects modern reassessments rather than the chronicles. Juvayni, Rashid al-Din and Ibn al-Athir give city totals that sum to well over five million and, in some readings, above ten; John Masson Smith's analysis of siege logistics and city sizes and David Morgan's source criticism treat these as impossible, leaving a very large but unquantified toll of which the range here is a conservative modern bracket.
The killing of urban populations, the selection and deportation of artisans, the use of captive levies (hashar) in assaults and the destruction of cities and irrigation works are documented by Juvayni, Rashid al-Din and Ibn al-Athir and assessed in the modern scholarship cited.
Turkmenistan, Uzbekistan, Mongolia, Afghanistan