刘黑闼
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 Shimin defeated Dou, capturing him. Despairing, Wang also surrendered, and most of the Zheng territory was seized by the Tang. Xia territory was also seized by Tang, but after Emperor Gaozu executed Dou, Dou's general Liu Heita rose against the Tang and seized most of the former Xia territory, while Xu Yuanlang, a rebel leader who had previously submitted to Zheng, also rose in revolt, occupying the modern Shandong region.