窦建德
Meanwhile, around the same time, the Tang general Li Shimin (Emperor Gaozu's son) was leading a major attack on Zheng's capital Luoyang. Wang Shichong, unable to fend off the attack by himself, sought aid from Dou Jiande, even though Xia and Zheng had previously had a poor relationship. Dou's official Liu Bin () suggested that he aid Zheng, reasoning that if Tang destroyed Zheng, Xia would be under Tang's threat thereafter—and that if he were victorious, he could then consider seizing Zheng territory. Dou agreed and sent messengers to Wang, promising support, while sending his official Li Dashi to Li Shimin, requesting that he end his campaign against Luoyang. Li Shimin, however, detained Li Dashi and did not respond.