In July of the third year of Wude, the rear area of Guanzhong in Datang was pacified, and Li Yuan sent Qin Wang as the commander in chief, leading various armies to March into Kanto, with Wang bearing the brunt. Our army is roughly the same as Wang's, with about 150 thousand people. Li Shimin is a soldier, and Tang Jun's morale is high. He quickly cleared the periphery of Luoyang and surrounded the king in Luoyang.

At this time, although Wang still had strong military strength, he was short of food and grass. It was not a good idea to confront Tang Jun with high morale. So the king couldn't hold on any longer, so as to defeat Tang Jun's spirit. On the one hand, he turned to Dou Jiande for help, in order to defeat Tang Jun from within. After getting Wang's help, Dou Jiande weighed it carefully. If Wang is defeated, then his next goal must be himself. The so-called lips die and teeth are cold. So Dou Jiande readily agreed to the king's request.

However, although Dou Jiande promised to come down, his mind was full of the idea that the fisherman would benefit. Because at this time, the main force of the king has been deadlocked for a long time, both sides not only lost a lot of fighting power and military forces, but also reinforced themselves when both sides were tired, not only destroying Tang Jun, but also destroying the king by the way. It wasn't until Wang couldn't go on, that Dou Jiande organized150,000 troops to reinforce.

Knowing the news of Dou Jiande's support, Wang strengthened his determination to defend the city. Tang Jun, on the other hand, suffered a long-term blow, and the Tang Jun period gradually declined, resulting in opposition from the DPRK. Tang Gaozu also believes that Dou Jiande should temporarily avoid withdrawing. However, I think that Wang is a spent force. As long as he is cut off from Dou Jiande, he can concentrate his forces on attacking Dou Jiande after Wang's rout.

So they seized Hulao Pass and cut off the connection between Dou Jiande and the king. But we need to continue to surround Wang, so we can only send a small number of troops to deal with Dou Jiande, so Dou Jiande has an absolute advantage. However, Hulao Pass was occupied by Li Shimin, and it was easier to defend than to attack. Dou Jiande's150,000 troops stormed for a month, but it still had no effect. Later, Li Shimin got the news of the spy. Dou Jiande wanted to sneak attack on Hulao Pass while Tang Jun was herding horses in Hebei, so Li Shimin played along and took Dou Jun Huanglong as a prisoner of Dou Jiande. Li Shimin, who returned in triumph, was also named "General Ce Tian" by Li Yuan.