in winter 641, Emperor Taizong promoted him to the post of minister of defense. Soon, however, even before Li Shiji could leave for Chang'an, Xueyantuo's Zhenzhu Khan Yi'nan, believing that Emperor Taizong was about to carry out a major sacrifice to the gods of Mount Tai and therefore would not have time for military action, launched a major attack against Tang's vassal, the Qilibi Khan Ashina Simo, whom Emperor Taizong had created the khan of a reestablished Eastern Tujue in 639. Ashina Simo could not resist, and withdrew within the Great Wall to Shuo Prefecture (roughly modern Shuozhou, Shanxi). Emperor Taizong launched armies, commanded by five generals, including Li Shiji, to aid Eastern Tujue. Around the new year 641, Li Shiji engaged Xueyantuo forces, under the command of Yi'nan's son Dadu (), at Nuozhen River (, flowing through modern Baotou, Inner Mongolia), defeating Xueyantuo forces and forcing them to flee. (Based on comments that Li Shiji made in 644 while advocating a campaign against Goguryeo, it appeared that Li Shiji wanted to advance further to try to destroy Xueyantuo, but Emperor Taizong, at the urging of Wei Zheng, ordered him to stop.) Li Shiji subsequently returned to Chang'an to serve as minister of defense.