Li Shimin, after defeating Liu, had started a campaign against Wang's Zheng state in fall 620.

He initially could not decisively defeat Zheng, but by spring 621 has put the Zheng capital Luoyang under a tight siege, although he is not able to capture it.

Wang seeks aid from Dou.

The latter agrees, concerned that a Tang victory over Zheng would also mean his own demise, but at the same time is eager to exploit the weakness of the Zheng and claim its domains for himself.

Emperor Gaozu is initially fearful that Dou and Wang would be able to sandwich Li Shimin's forces between them and orders Li Shimin to retreat, but upon Li Shimin's petition changes his mind and permits Li Shimin to remain in the Luoyang region.

Li Shimin, leaving Li Yuanji in charge of the siege of Luoyang, advances and takes up a position at Hulao Pass.

On May 28, 621, the Tang and Xia forces engage at Hulao, and Li Shimin defeats Dou, capturing him.

Despairing, Wang also surrenders, and most of the Zheng territory is seized by the Tang.