刘黑闼
The Eastern Turkic Khaganate was a vassal of the Sui Empire - the Tang's predecessor - but had rebelled during the reign of Emperor Yang of Sui (r. 604-618). During the Sui breakup late in Yang's reign, the Khaganate supported various claimants to the imperial title to promote conflict and make them subservient to the Khaganate. Among those benefiting from Khaganate support were Emperor Gaozu (founder of the Tang), Xue Ju (Emperor of Qin), Liang Shidu (Emperor of Liang), Liu Wuzhou (Dingyang Khan), Gao Kaidao (Prince of Yan), Dou Jiande (Prince of Xia), and Liu Heita (Prince of Handong). The Tang won hegemony in 623 following the death of Liu Heita. Following the Tang victory, the Khaganate began asserting control over Tang territory through routine raids even though Gaozu remained an ally and tributary. Furthermore, the Khaganate continued to support Liang Shidu (the last competing Chinese claimant to Tang), and harbour Prince Yang Zhengdao and his grandmother Empress Xiao of Sui. The Turkic raids were so serious that Gaozu considered moving the Tang capital from Chang'an to what is now southwestern Henan. The move was supported by crown prince Li Jiancheng, Li Yuanji (Prince of Qi), and the chancellor Pei Ji. Li Shimin, the Prince of Qin and future Emperor Taizong, convinced Gaozu to abandon the proposal by promising to - eventually - defeat the Eastern Turks.