Li Shimin learned that Dou Jiande led hundreds of troops to come to the rescue. In fact, this has long been in his battle plan, which can be implemented in turn according to the original plan: First, the main forces continue to surround Luoyang, but they do not take the initiative to attack the city, but only surround the troops in the city. The defenders in the city are also prepared to defend the city in case reinforcements attack the siege troops, so they haven't considered other ways, such as breaking through, and naturally let Tang Jun besiege. On the other hand, Li Shimin brought its own elite troops to Hulao Pass to help. The essence of this siege is that the defenders and reinforcements can't meet each other, because meeting will boost morale and increase the difficulty of siege, and of course they can't meet each other. So, Li Shimin kept the pass two or three hundred miles away around Luoyang, separating the besieged city from Dayuan battlefield, so as to avoid the defenders meeting reinforcements. There are many siege wars in history because the banner of reinforcements appeared in the field of vision of the defenders in Chengtou, which revived the morale of the defenders and led to failure. Of course, the patentee of siege Dayuan tactics did not make such a mistake. He took the initiative to go to Hulao Pass, Luoyang City, where the naked eye could not face the reinforcements.
How good is this Hulao Pass? How can it be easily broken? Dou Jiande was eager to save Luoyang City, so he was eager to storm the elite Tang Jun which occupied the favorable terrain in Hulao Pass. Dou Jiande didn't read The Romance of the Three Kingdoms written by Luo Guanzhong in the Ming Dynasty, so he didn't know that an archer like Guan Yu was needed to attack Hulao Pass. Li Shimin's men didn't go out to make up for Hua Xiong, so Dou Jiande's fiasco is understandable. In the end, Dou Jiande was completely annihilated, and I became a prisoner of Li Shimin. When the defenders of Luoyang City waited for reinforcements for many days, they suddenly saw their long-awaited sinus savior tied up and appeared in front of Li Shimin's horse. Morale suddenly collapsed, and failure was inevitable. Therefore, this campaign is listed as a typical tactic of "siege warfare" in military history, which is talked about by later generations and is also a tactic often used by military strategists in modern countries when necessary. (Quoted from "Iron Blood Strong Song")