Emperor Taizong of Tang
Campaign against the Eastern Turks

In 628, with Ashina Duobi and Ashina Shibobi having a falling out, Ashina Shibobi submitted to Emperor Taizong, as did the chieftains of Khitan tribes, who had previously submitted to the Eastern Turks. With their khaganate in turmoil, Ashina Duobi was no longer able to protect the last late-Sui rebel ruler who alone remained standing against Tang pressure— Liang Shidu the Emperor of Liang, and in summer 628, with the Tang generals Chai Shao and Xue Wanjun (薛萬均) sieging the Liang capital Shuofang (in modern Yulin, Shaanxi), Liang Shidu's cousin Liang Luoren (梁洛仁) killed Liang Shidu and surrendered, finally uniting China. With the Eastern Turkic Khaganate weakened, their vassal Xueyantuo also broke away and formed its own khanate, and Emperor Taizong entered into an alliance with Xueyantuo's leader Yinan, creating Yinan the Zhenzhupiqie Khan (or Zhenzhu Khan in short).

In late 629, believing the time ripe for a major attack on the Eastern Turks, Emperor Taizong commissioned the general Li Jing with overall command of a multi-pronged army, assisted by the generals Li Shiji, Li Daozong, Chai Shao, Xue Wanche (薛萬徹, Xue Wanjun's brother) and Su Dingfang, attacking the Eastern Turks at multiple points. The army was successful in its attacks, forcing Ashina Duobi to flee, and by late spring 630, Ashina Duobi had been captured, and Eastern-Turkic chieftains all submitted to Tang. Emperor Taizong spared Ashina Duobi but detained him at Chang'an, and he considered what to do with the Eastern Turks.

The main opposing views were from the chancellors Wen Yanbo (who advocated leaving the Turks within China's borders to serve as a defense perimeter) and Wei (who advocated leaving them outside the borders). Emperor Taizong accepted Wen's suggestion and established a number of prefectures to accommodate the Turks, leaving them governed by their chieftains without creating a new khan to govern them.

In 631, Emperor Taizong established a feudal scheme, where the contributors to his reign were given, in addition to their current posts, additional posts as prefectural governors, to be passed on to their descendants. Soon, however, receiving much opposition to the plan, the strongest of which came from Zhangsun Wuji, Emperor Taizong cancelled the scheme.After the conquest of the Eastern Turkic Khaganate, Emperor Taizong's officials repeatedly requested that he carry out sacrifices to heaven and earth at Mount Tai, and Emperor Taizong, while at times tempted by the proposal, was repeatedly dissuaded from doing so by Wei, who pointed out the expenses and the labors that would be imposed on the people as a result, and also that this would open China's borders to attack.
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