The ninth century was marked in Transoxania, as we have said, by a de facto reaffirmation of Central Asian identity at the expense of the Arab conquerors. For the time being, this identity, while Islamic religiously and culturally, was chiefly Iranian, despite the strengthening of Turkic elements. In Sinkiang, on the other hand, there occurred the first of the events that opened the gate toward the eventual Turkicization of the greater part of Inner Asia. It was triggered by another event in Mongolia whose importance lay mainly in setting this process in motion.

The Uighur qaghanate of Mongolia was in 840 overthrown by the Kyrgyz, a Turkic people who at that time lived in southern Siberia along the upper Yenisei and in what is today the republic of Tuva, where the Yenisei has its sources. Most of the Uighurs then moved west, chiefly to two areas: to the Chinese province of Kansu, where they became known as Yellow Uighurs but otherwise did not leave much trace; and to eastern Sinkiang, where they founded the Uighur kingdom of Qocho, which would last for four centuries (850–1250).

As we have said in the introductory chapter, Qocho is today an archeological site, situated about 30 kilometers to the east of Turfan; Turfan, in turn, is flanked on the west by another ruin, a still older city known by its Chinese name of Chiao-Ho (“City [between] two rivers”) and, later, by its Turco-Mongol name Yar-khoto (“Cliff city”).