While the Shah planned to defend his major cities individually and divided his army to station in several garrisons, the Mongols laid siege to one town after another deep into Khorasan, heart of the Khwarazmian Empire.

A Mongol force, estimated to number between 30,000 and 50,000 men and led by Tolui, son of Genghis Khan, traversed the Karakum Desert after destroying the former imperial capital Gurganj in the north.

Historical accounts contend that Merv's entire population, including refugees, who had previously fled from other besieged towns of the empire, were killed.

Mongols are reputed to have slaughtered 700,000 people,[2][3][4] while Persian historian, Juvayni, as well as R. J. Rummel put the figure at more than 1,300,000,[5][6] making it one of the bloodiest captures of a city in world history.

[8][9][10] Around this time, Merv turned into a chief centre of Islamic science and culture, attracting as well as producing renowned poets, musicians, physicians, mathematicians and astronomers.

[16] The Mongol armies arrived in Khwarazmia in waves: first, a vanguard led by Genghis' eldest son Jochi and the general Jebe crossed the Tien Shan passes, and started laying waste to the towns of the eastern Fergana Valley.

[17] Genghis soon arrived with his youngest son Tolui, and split the invasion force into four divisions: while Chagatai and Ogedai were to remain besieging Otrar, Jochi was to head northwest in the direction of Gurganj.

[20] The Shah distrusted most of his commanders, the only exception being his eldest son and heir Jalal al-Din, whose military skill had been critical at the Irghiz River skirmish the previous year.

[24] Arab historian Ibn al-Athir described the event basing his report on the narrative of Merv refugees: Genghis Khan sat on a golden throne and ordered the troops who had been seized should be brought before him.

They took the wealthy people and beat them and tortured them with all sorts of cruelties in the search for wealth ... Then they set fire to the city and burned the tomb of Sultan Sanjar and dug up his grave looking for money.

"[25] Almost the entire population of Merv, and refugees arriving from the other parts of the Khwarazmian Empire, were slaughtered, making it one of the bloodiest captures of a city in world history.

[26] Some time later in 1221, Khwarazmshah Jalal ad-Din attacked a detachment of Mongols near Wilan, which provoked Genghis Khan into sending an army of 30,000 troops under Shigi Qutuqu.