However, in 644, faced with constant pressure from Xueyantuo, Ashina Simo's people abandoned him and fled south back to Tang territory.
According to Suzuki Kosetsu, it was a political situation created by Issik Khagan's descendants, who did not want succession crisis.
In spring 641, Ashina Simo's people finally crossed the Yellow River, and he established his headquarters at Dingxiang (定襄, in modern Hohhot, Inner Mongolia).
He submitted a petition to Emperor Taizong, stating: I have received blessings that I did not deserve, and I lead this tribe as its chief.
In winter 641, indeed, with Yi'nan anticipating that Emperor Taizong would soon be offering sacrifices to heaven and earth at Mount Tai and would take his soldiers with him, he believed that he could destroy Ashina Simo quickly.
Ashina Simo could not resist the attack, and withdrew within the Great Wall to Shuo Prefecture (朔州, roughly modern Shuozhou, Shanxi) and sought emergency aid.
(Yi'nan, however, tried to maintain peaceful relations with Tang, and at one point was engaged to marry Emperor Taizong's daughter Princess Xinxing, but Emperor Taizong regretted the marriage agreement and broke it in 643 under the pretense that Yi'nan's offer of bride price (with livestock) was not paid for on time.)
During siege of Baekam (백암성), Ashina Simo was injured by an arrow, and it was said that Emperor Taizong personally sucked the pus out of his wound, which greatly inspired the soldiers.
[5] Taizong himself also tended to the injury of another Turkic General who was wounded during the war against Goguryeo, Qibi Heli.
[6] It was said that after Ashina Simo returned to the Tang capital Chang'an (implying that this is after the end of the campaign in fall 645), he died.
Thus although he always held the title of Jiabi Teqin [夾畢特勤; ms. 夾畢特勒][a] during Chuluo and Xieli’s time, he could not become a shad (she 設) in command of the army till the end.