determined to march south against Tang. Emperor Gaozu sent Pei Ji against Liu's advancing army, but Pei was defeated by Liu, who then put Taiyuan under siege. Li Yuanji fled back to Chang'an, and much of modern Shanxi was seized by Liu. Emperor Gaozu then sent Li Shimin against Liu, and by summer 620, Li Shimin had defeated Liu, forcing him to flee to the Eastern Turks. Liu's territory was incorporated into Tang. Around the same time, however, Dou Jiande the Prince of Xia made 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 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 () captive. With Li Gai in Dou's custody, Li Shiji surrendered to Dou as well. In 620, Li Shiji, in association with another Tang general who surrendered to Dou, Li Shanghu (), plotted to ambush Dou, but the plot was discovered; Li Shanghu was killed, and Li Shiji fled back to Tang. In 620, Li Fuwei captured much of the territory of another agrarian ruler, Li Zitong, the Emperor of Wu, in the lower Yangtze River region, in the name of Tang Dynasty. Li Zitong, in turn, defeated and took over the territory of Shen Faxing the Prince of Liang, roughly modern Zhejiang. After Li Shimin defeated Liu, he started a campaign against Wang's Zheng state in fall 620. He initially could not decisively defeat Zheng, but by spring 621 had put the Zheng capital Luoyang under a tight siege, although he was not able to capture it. Wang sought aid from Dou. The latter agreed, concerned that a Tang victory over Zheng would also mean his own demise, but at the same time was eager to exploit the weakness of the Zheng and claim its domains for himself. Emperor Gaozu was initially fearful that Dou and Wang would be able to sandwich Li Shimin's forces between them and ordered Li Shimin to retreat, but upon Li Shimin's petition changed his mind and permitted Li Shimin to remain in the Luoyang region. Li Shimin, leaving Li Yuanji in charge of the siege of Luoyang, advanced and took up position at Hulao Pass. In summer 621, the Tang and Xia forces engaged at Hulao, and Li