The Mongol invasion of Central Asia occurred after the unification of the Mongol and Turkic tribes on the Mongolian plateau in 1206. It finally completed when Genghis Khan conquered the Khwarizmian Empire in 1221.
The Uyghurs, Qarluqs and local Turkic and Tajik peoples submitted to the Mongolians. The Uyghur state of Kara-Khoja was a vassal of the Kara-Khitans, but in 1210, the Uyghur ruler of Kara-Khoja, Idiqut Barchuq appeared before the Khan to declare his allegiance to the Mongolians. He was rewarded with the daughter of Genghis in marriage, and the Uyghurs served under the Mongols as bureaucrats. A leader of the Qarluq and Buzar, the warlord of Chuy Valley, followed the Uyghur example.
Kuchlug attacked the city of Almaliq, and the Qarlugs there who were vassal of the Mongolians appealed to Chinggis Khaan for help. In 1216, Genghis dispatched his general Jebe to pursue Kuchlug. The Mongols defeated the Kara-Khitans at Balasaghun, Kuchlug fled, but was later killed in 1218.
The Mongol invasion of Khwarezmia lasted from 1219 to 1221. In fact, it was not originally the intention of the Mongol Empire to invade the Khwarezmid Empire. According to Juvaini, Genghis Khan had originally sent the ruler of the Khwarezmid Empire, Sultan Muhammad Aladdin, a message greeting him as his equal: "We now have the obligations of neighbors. Human wisdom requires that both sides walk the path of concord and observe the duties of friendship." The Mongols' original conquest of all "people in felt tents", unifying the nomadic tribes in Mongolia and then the Turcomens and other nomadic peoples, had come with relatively little bloodshed, and almost no material loss. However, the Governor of Otrar refused to receive the mission and had all 450 of them killed with permission from the Sultan. Upon hearing of this atrocity months later, Genghis Khan flew into a rage and used the incident as a pretext for invasion.
The Mongol invasion of Central Asia however would entail the utter destruction of the Khwarezmid Empire along with the massacre of much of the civilian population of the region. According to Juvaini, the Mongols ordered only one round of slaughter in Khwarezm and Transoxiana, but systematically exterminated a particularly large portion of the people of the cities of Khorasan. This earned the Mongols a reputation for bloodthirsty ferocity that would mark the remainder of their campaigns.
During the invasion of Transoxania in 1219, along with the main Mongol force, Genghis Khan used a Chinese specialist catapult unit in battle. They were used again in 1220 in Transoxania. The Chinese may have used the catapults to hurl gunpowder bombs, since they already had them by this time  While Genghis Khan was conquering Transoxania and Persia, several Chinese who were familiar with gunpowder were serving with Genghis's army. Historians have suggested that the Mongol invasion had brought Chinese gunpowder weapons to Central Asia. One of these was the huochong, a Chinese mortar.
↑Svatopluk Soucek (2000). "Chapter 4 - The Uighur Kingdom of Qocho". A history of Inner Asia. Cambridge University Press. ISBN 0-521-65704-0.
↑Biran, Michal. (2005). "Chapter 3 - The Fall: between the Khwarazm Shah and the Mongolians". The Empire of the Qara Khitai in Eurasian History: Between China and the Islamic World. Cambridge University Press. pp. 60–90. ISBN 0-521-84226-3.
↑Svatopluk Soucek (2000). "Chapter 6 - Seljukids and Ghazvanids". A history of Inner Asia. Cambridge University Press. ISBN 0-521-65704-0.
↑Biran, Michal. (2005). The Empire of the Qara Khitai in Eurasian History: Between China and the Islamic World. Cambridge University Press. pp. 84–85. ISBN 0-521-84226-3.