Emperor Taizong of Tang
Campaign against Western Turks

Taizong began a series of campaigns against the Western Turks (Xitujue) and their allies, the oasis states of the Tarim Basin, around 640. These hostilities between the Western Turks and Tang would continue until the defeat and conquest of the Western Turks in 657 under Taizong's successor, Gaozong. The kingdoms of Shule and Khotan surrendered to the Chinese in 632, as did the kingdom of Yarkand in 635. Qu Wentai (麴文泰), the king of Gaochang, who had previously been submissive to Tang, had become increasingly hostile to Tang, allying with the Western Turks. In 640, Emperor Taizong commissioned Hou Junji, assisted by Xue Wanjun, to launch a major attack on Gaochang. As they approached Gaochang, Qu Wentai died in fear and was succeeded by his son Qu Zhisheng (麴智盛). Qu Zhisheng offered to submit, but Hou demanded a surrender, which Qu Zhisheng refused. However, Hou put Gaochang under siege, and with aid from the Western Turks not arriving, Qu Zhisheng surrendered. Wei Zheng suggested that Emperor Taizong allow Qu Zhisheng to remain king, pointing out that the monetary and human costs would be high to keep a permanent garrison at Gaochang, but Emperor Taizong disagreed, and he converted Gaochang into two prefectures and annexed it into his state.

In winter 640, Songsän Gampo sent his prime minister Gar Tongtsen Yülsung ("Lu Dongzan" (祿東贊) in Chinese) as an emissary to Tang, offering tributes and again requesting marriage. Emperor Taizong created a daughter of a clansman as the Princess Wencheng, and in 641 sent Li Daozong to accompany Princess Wencheng to Tibet to preside over the wedding.In winter 641, believing that Emperor Taizong was about to carry out sacrifices to heaven and earth at Mount Tai and would be unable to aid the Eastern Turks, Yinan launched a major attack on them, commanded by his son Dadu (大度). Li Simo was forced to retreat inside the Great Wall. Emperor Taizong commissioned Li Shiji, assisted by Zhang Jian (張儉), Li Daliang, Zhang Shigui (張士貴), and Li Xiyu (李襲譽), to attack Xueyantuo. Li Shiji soon defeated Dadu at Nuozhen River (諾真水, flowing through modern Baotou, Inner Mongolia), and Dadu fled.
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