Emperor Taizong of Tang (r. 626—649), the second emperor of Chinese dynasty Tang Dynasty, early in his reign, had allied with Xueyantuo, a vassal of the powerful Eastern Tujue (Göktürk) Khanate, against Eastern Tujue, who Tang defeated in 630. Upon Eastern Tujue's defeat, Xueyantuo's Zhenzhu Khan Yi'nan took over Eastern Tujue's former territory, and while he was formally submissive to Tang, he was expanding his own strength. When Emperor Taizong tried to restore Eastern Tujue in 639 under the Qilibi Khan Ashina Simo (also known as Li Simo) to counteract the rise of Xueyantuo power, Xueyantuo engaged in multiple battles with the newly restored Eastern Tujue, in order to prevent this return. The major Tang general Li Shiji temporarily came to protect Eastern Tujue against Xueyantuo and defe