Tang is meanwhile facing another serious threat—Liu Wuzhou, now determined to march south against Tang.

Emperor Gaozu sends Pei Ji against Liu's advancing army, but Pei is defeated by Liu, who then puts Taiyuan under siege.

Li Yuanji flees back to Chang'an, and much of modern Shanxi is seized by Liu.

Emperor Gaozu then sends Li Shimin against Liu, and by summer 620, Li Shimin has defeated Liu, forcing him to flee to the Eastern Turks.

Liu's territory is incorporated into Tang.

Around the same time, however, Dou Jiande the Prince of Xia makes a major offensive against the cities that had submitted to Tang in modern Hebei and Henan, north of the Yellow River, seizing nearly all of them and taking captive Emperor Gaozu's cousin Li Shentong the Prince of Huai'an, Emperor Gaozu's sister the Princess Tong'an, and Li Shiji's father Li Gai.

With Li Gai in Dou's custody, Li Shiji surrenders to Dou as well.

In 620, Li Shiji, in association with another Tang general who had surrendered to Dou, Li Shanghu, plots to ambush Dou, but the plot is discovered; Li Shanghu is killed, and Li Shiji flees back to Tang.