The conquest of the Western Turks, known as the Western Tujue in Chinese sources, was a military campaign in 655–657 led by the Tang dynasty generals Su Dingfang and Cheng Zhijie against the Western Turkic Khaganate ruled by Ashina Helu. The Tang campaigns against the Western Turks began in 640 with the annexation of the Tarim Basin oasis state Gaochang, an ally of the Western Turks. Several of the oasis states had once been vassals of the Tang dynasty, but switched their allegiance to the Western Turks when they grew suspicious of the military ambitions of the Tang. Tang expansion into Central Asia continued with the conquest of Karasahr in 644 and Kucha in 648. Cheng Zhijie commanded the first foray against the West Tujue, and in 657 Su Dingfang commanded the main army dispatched against the Western Turks, while the Turkic generals Ashina Mishe and Ashina Buzhen led the side divisions. The Tang troops were reinforced by cavalry supplied by the Uyghurs, a tribe that had been allied with the Tang since their support for the Uyghur revolt against the Xueyantuo. Su Dingfang's army defeated Helu at the battle of Irtysh River.
The victory strengthened Tang control of the Western Regions, now modern Xinjiang, and brought the regions formerly ruled by the Khaganate into the Tang empire. Puppet qaghans, the Turkic title for ruler, and military garrisons were installed to administer the newly acquired territories. The Tang dynasty achieved its maximum territorial extent as its western borders reached the eastern frontier of the Umayyad Caliphate. Later on, Turkic revolts ended Tang hegemony beyond the Pamir Mountains in modern Tajikistan and Afghanistan, but a Tang military presence remained in Dzungaria and the Tarim Basin. Central Asia absorbed cultural influences from the conflict. Turkic culture and language spread into Central Asia, as did artistic and political influences from the Tang dynasty. Many of the Tang generals and soldiers stationed in the region were ethnically Turkic, and the prevalence of Indo-European languages in Central Asia declined with acceleration of Turkic migration. The Turks, Tibetans, and the Tang competed for control over Central Asia for the next few centuries.
Background
The empire of the Tang dynasty (June 18, 618 – June 1, 907), successor of the Sui dynasty, was a cosmopolitan hegemon that ruled one of China's most expansive empires. Raids by the nomadic Khitans and Turks challenged Tang rule, and Tang rulers responded by pursuing strategies of divide and conquer, proxy warfare, tributes, and marriages.
Hostilities between the Tang and the Western Turks had existed since the founding of the dynasty. Emperor Gaozu, the first emperor of the Tang dynasty, aided the assassination of a Western Turk qaghan on November 2, 619. Facing the threat of both the Western and Eastern Turkic Khaganates, Gaozu's successor Emperor Taizong formed an alliance with the Western Turks against the Eastern Turks, adopting a policy of allying "with those who are far away to fight those who are close."
The westward expansion of the Tang dynasty began with their wars against the Eastern Turks, Eastern Tujue in Chinese. Taking advantage of the political discord in the Eastern Turkic Khaganate, Taizong annexed the territory of the Eastern Turks in 629, beginning a period of rule that would last for the next fifty years. The nomads were driven out of the Ordos region and southern Mongolia and Taizong was declared a Great Khan by the defeated tribes, who surrendered and submitted to Tang rule.
Battles in the Tarim Basin
Several of the Tarim Basin oasis states switched their allegiance from the Tang dynasty to the Western Turks. The oasis states Kashgar and Khotan surrendered to the Chinese in 632, as did the kingdom of Yarkand in 635. Tang military campaigns expanded further west against the remaining kingdoms of the Tarim Basin in southern Xinjiang beginning in 640. The king of Gaochang refused to submit to the Tang dynasty as a suzerain. In 638, Emperor Taizong ordered a campaign led by general Hou Junji to invade Gaochang. Tang troops arrived in 640 and annexed the kingdom. A Western Turk army, sent to support Gaochang, retreated as the Tang forces approached.