However, Yi'nan disliked the situation and, later that year, believing that Emperor Taizong was about to offer sacrifices to heaven and earth at Mount Tai and would take his soldiers with them, he wanted to use this chance to destroy Ashina Simo. He commissioned his son Dadu (), with an army made of soldiers from the Tongluo, Pugu, Huige, Mohe, and Xi, to attack Eastern Tujue with 200,000 men. Ashina Simo could not resist, and withdrew within the Great Wall, took up position at Shuo Prefecture (roughly modern Shuozhou, Shanxi), and sought emergency aid from Emperor Taizong. In winter 641, Emperor Taizong sent the generals Zhang Jian, Li Shiji, Li Daliang, Zhang Shigui (), and Li Xiyu (), to attack Xueyantuo to try to save Ashina Simo. Around the new year 642, Li Shiji engaged Dadu's army. Dadu's army was initially able to kill Li Shiji's army's horses with arrows, but Li Shiji's lieutenant Xue Wanche () was able to, in turn, concentrate on attacking Xueyantuo army's horses. Li Shiji was able to thereafter defeat Dadu, forcing him to flee. (At the same time, Yi'nan had sent an emissary to Emperor Taizong, offering peace with Eastern Tujue, and after Li Shiji defeated Dadu, Emperor Taizong sent the emissary back to Yi'nan with harsh words, but did not make further attacks on Xueyantuo or rebuke Yi'nan any further.)