Emperor Taizong's campaign against Xueyantuo
From Warlike
Early in his reign Emperor Taizong of Tang, had allied with Xueyantuo, a vassal of the Eastern Turkic Khaganate, against the Eastern Turks, who the Tang defeated in 630. Upon the Eastern Turks' defeat, the Xueyantuo's Zhenzhu Khan took over the Eastern Turks' former territory, and while he was formally subservient to the Tang, he was expanding his own strength. When Emperor Taizong tried to restore the Eastern Turks in 639 under Qilibi Khan to counteract the rise of Xueyantuo power, the Xueyantuo engaged in multiple battles with the newly restored Eastern Turks, to prevent this return. The major Tang general Li Shiji temporarily came to protect the Eastern Turks against the 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 the Eastern Turks, forcing Qilibi Khan to flee back to the Tang. Subsequently, Goguryeo sought aid from the Xueyantuo, but Zhenzhu avoided further conflict, wanting to avoid direct battle with Tang. After Zhenzhu'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 the Uyghurs rose and killed him. His cousin, the Yitewushi Khan Duomozhi, surrendered to Tang forces, ending Xueyantuo.
