Emperor Taizong's campaign against Xueyantuo
At some point, the Chile became submissive to Eastern Turks instead, and as of the reign of the Jiali Khan Ashina Duobi, the Xueyantuo were ruled by Yishibo's grandson Yi'nan.
As of 641, Ashina Simo had finally marched north of the Great Wall and settled in at Dingxiang (定襄, in modern Hohhot, Inner Mongolia).
Ashina Simo withdrew within the Great Wall and took up position at Shuo Prefecture (roughly modern Shuozhou, Shanxi) and sought emergency aid from Tang.
Meanwhile, Yi'nan had sent an emissary to Emperor Taizong to offer peace with Eastern Turks, and Li Shiji prevailed over Dadu, Emperor Taizong sent the emissary back with a rebuke for Yi'nan but did not further take actions against Xueyantuo, and thereafter, the relationship between Tang and Xueyantuo officially remained that of lord and vassal, but appeared to be no longer as strong as it had been.
In 643, Yi'nan again sent his nephew Tuli (突利) to offer tributes of 50,000 horses, 10,000 cattle or camels, and 100,000 goats, to serve as bride price.
When Yi'nan agreed to go to Ling Prefecture, Emperor Taizong found another excuse—that the bride price offered had not been all collected (as, in order to gather the livestock making up the bride price, Yi'nan had to collect them from subordinate tribes, and it was taking longer than thought, and the livestock were also dying from having to go through the Gobi) -- to cancel the marriage treaty, despite strong opposition from his official Chu Suiliang, who pointed out that, effectively, he was devaluing his own words.
Emperor Taizong rationalized his decision by arguing that if Yi'nan had married a Tang princess, he would have greater legitimacy over the Chile tribes and would be more difficult to control.
I am willing to kill the Turks for China.By the end of 644, the Eastern Turkic people, who were not whole-heartedly supportive of Ashina Simo in the first place, collapsed in light of Xueyantuo threat, fleeing back to Tang territory, and were again settled there.
However, Emperor Taizong, anticipating the possibility of a Xueyantuo attack, had had the general Zhishi Sili (執失思力) command Turkic soldiers to defend Xia Prefecture (夏州, roughly modern Yulin, Shaanxi).
Briefly after the new year 646, Emperor Taizong ordered, in addition to Zhishi and Tian's troops, for troops to be mobilized under the generals Li Daozong the Prince of Jiangxia, Xue Wanche (薛萬徹), Ashina She'er (阿史那社爾), Song Junming (宋君明), and Xue Guwu (薛孤吳), to defend against Bazhuo's attack, which Bazhuo called off after reaching the Great Wall and realizing that Tang forces had been mobilized.
In turn, in spring 646, Zhishi and Qiao Shiwang counterattacked, defeating Bazhuo and forcing him to flee, throwing Xueyantuo into a state of confusion.
He removed Yi'nan's chief advisors and replaced them with people close to him, which led to the nobles despising him—and he responded by killing a large number of them, throwing the Xueyantuo court into terror.
Yaoluoge Tumidu (藥羅葛吐迷度), the chieftain of Huige, then a Xueyantuo vassal, rebelled along with the Pugu (僕骨) and Tongluo (同羅) tribes and dealt Bazhuo a great defeat.
Emperor Taizong took the chance to order a major assault against Xueyantuo proper by Li, Ashina, Zhishi, Qibi Heli, Xue Wanche, and Zhang Jian.
Yuwen attacked Abo with the Mohe troops with him at the time and defeated Abo—which led to even greater confusion for Xueyantuo, whose people believed that the main Tang army had already arrived.
