First of all, Dou Jiande's ulterior motives gave Li Shimin a lead in this war. After the king was besieged in Luoyang, he turned to Dou Jiande for help, but Dou Jiande did not send troops immediately after getting help. For Dou Jiande, and Wang will be enemies sooner or later. He sent troops to save the king to prevent him from becoming too powerful. What he thinks in his mind is to slow down the rescue and let both sides consume more troops. In this way, when he arrives, he will take advantage of the fisherman.
It is with this in mind that Dou Jiande's army moves very slowly. On the other hand, when Li Shimin decided to stay and continue to conquer Dou Jiande, he took the elite soldiers to Hulao Pass non-stop. At that time, not all Hulao Pass was under the control of Tang Jun, but Hulao Pass's position and terrain determined his strategic position. It is said that Li Shimin and all the soldiers didn't dismount for days and nights, just to seize the initiative. As a result, Li Shimin not only entered Hulao Pass before Dou Jiande, but even because of Dou Jiande's delay, it was too late to rest and strengthen the defense. Later, Dou Jiande's army never advanced in Hulao Pass, which also proved Li Shimin's strategic vision.
In addition, Dou Jiande's underestimation is also an important reason for his failure. When he learned that Li Shimin led more than 3,000 soldiers to attack him in Hulao Pass, he didn't care. He and his men discussed how to fight Tang Jun's main force. It was not until he really fought Li Shimin that he found himself unprepared for the war. Subsequently, Dou Jiande and Li Shimin fell into a long-term confrontation.
In this case, the morale of soldiers on both sides has changed. Dou Jiande's army is based on farmers, and the soldiers brought by Li Shimin are the elite. During the long confrontation, the soldiers in Dou Jiande began to feel homesick and their morale was very low. Li Shimin's army clearly knows that the closer it is to victory, the more persistent it will be. So after a positive success, Dou Jiande was captured and his 100,000-strong army went up in smoke.