Emperor Taizong's campaign against Xueyantuo

The Emperor Taizong of Tang (r. 626–649), the second emperor of the Tang dynasty, early in his reign, had allied with Xueyantuo, a vassal of the powerful Eastern Turkic Khaganate, against Eastern Turks, who Tang defeated in 630. Upon Eastern Turks' defeat, Xueyantuo's Zhenzhu Khan Yi'nan took over Eastern Turks' former territory, and while he was formally submissive to Tang, he was expanding his own strength. When Emperor Taizong tried to restore Eastern Turks 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 Turks, in order to prevent this return. The major Tang general Li Shiji temporarily came to protect Eastern Turks against Xueyantuo and defeated the Xueyantuo forces in 641. But in 644, with Emperor Taizong occupied with a campaign against Goguryeo, Xueyantuo forces launched a new campaign, defeated Eastern Turks, forcing Ashina Simo to flee back to Tang. Subsequently, Goguryeo sought aid from Xueyantuo, but Yi'nan avoided further conflict, wanting to avoid direct battle with Tang. After Yi'nan's death in 645, however, his son Duomi Khan Bazhuo began heavily battling Tang forces. In 646, Tang forces counterattacked, and after they defeated Bazhuo, Xueyantuo's vassal Huige rose and killed him. His cousin, the Yitewushi Khan Duomozhi, surrendered to Tang forces, ending Xueyantuo.