李世勣
Li Shiji soon considered how he could again submit to Tang, but worried that Dou would kill his father Li Gai. Guo Xiaoke suggested to him that he needed to first gain Dou's trust by accomplishing things for Xia. Li Shiji agreed, and in winter 619, he attacked the city of Huojia (in modern Xinxiang, Henan), held by Wang Shichong (who had by that point had Yang Tong yield the throne to him, ending Sui and establishing a new state of Zheng) and captured much goods and persons to present to Dou, including Dou's childhood friend Liu Heita. Dou began to trust him. Li Shiji then suggested to Dou that he should attack the agrarian leader Meng Haigong (), who was then nominally submitting to Zheng, arguing that if Xia could first capture Meng's holdings, it could then next have designs on Zheng. Dou agreed, and he sent his brother-in-law Cao Dan () south across the Yellow River, joining forces with Li Shiji. Dou himself would follow, and Li Shiji planned that, as soon as Dou himself arrived, he would ambush Dou's camp and kill him, and then try to find and save his father Li Gai. However, at this time, Dou was awaiting his wife Empress Cao's giving birth and did not arrive for a while. Meanwhile, Cao Dan was insulting and pillaging the other rebel leaders south of the Yellow River who had submitted to Xia, and the rebel leaders were all resentful. One of them, Li Shanghu (), and Li Shanghu's mother Lady Huo, urged Li Shiji to carry out his plan as soon as possible, and when Li Shiji hesitated, Li Shanghu and Lady Huo acted on their own, ambushing Cao Dan, but while they killed many of Cao's generals, Cao himself was not harmed and soon prepared to counterattack. Li Shanghu notified Li Shiji and asked him to attack Cao, but Li Shiji, saw that Cao had already taken precautions, fled to Tang territory with Guo. Cao soon defeated and killed Li Shanghu, but when Dou's officials suggested that Li Gai be executed, Dou remarked, "Li Shiji was a Tang subject. He was captured by us, but still remembered his former lord and was faithful. What sin did his father have?" Dou then spared Li Gai.