In 214, Liu Bei took Yizhou, and before he could digest and rectify Lisuo, Sun Quan demanded that Liu Bei return Nan Commandery on the grounds that Liu Bei had territory after capturing Yizhou and needed to return Jingzhou. Of course, Liu Bei did not agree, and the two sides were once sword-fighting, and Liu Bei personally led his troops to Jingzhou to confront Sun Quan. As a result, Boss Cao seized the opportunity at this time and led an army to attack Zhang Lu in Hanzhong and took Hanzhong in July. Liu Bei immediately made peace with Eastern Wu, and the two sides demarcated the Xiangshui Border, and Liu Bei returned Guiyang County and Changsha County to Sun Quan, and then immediately rushed to Hanzhong.
Why is Hanzhong so important? Hanzhong was the northern gateway of Shu, backed by Yizhou, and if Hanzhong was captured by Cao Cao, Yizhou would be difficult to secure. In addition, the "Huayang Tuzhi" says in Hanzhong: "Womei, Fu Gong, slightly three Shu". Since the ancient Han Dynasty, it has been rich and oily, agriculture is very developed, wealth is fertile, and it can feed hundreds of thousands of people. Therefore, For Liu Bei, who has been poor all his life, Hanzhong is a place of life and death, a lifetime of enduring humiliation and humiliation, and success or failure is in this move.
From 217 to 219 AD, Liu Bei mobilized the strength of the whole country, even to the point of men's war and women's luck, and finally broke Xiahou Yuan, tore open the gate of Hanzhong, and approached Hanshui. Cao Cao immediately reacted, personally came to Hanzhong to sit down, and even mobilized more than 300,000 people in Wudu and 80,000 people in Hanzhong in an attempt to destroy Liu Bei in one fell swoop. However, because of the huge logistical pressure, and For Cao Cao in Hanzhong, the food was tasteless and it was a pity to abandon it. So Cao Cao weighed it up again and again, and finally decided to withdraw from Hanzhong in 219 AD, and Liu Bei finally got the coveted Hanzhong.
Hanzhong is bordered by Guanzhong in the north and Xichuan in the south, with a dangerous geographical location and abundant products, which is a place where soldiers and families must compete. Liu Bei would let people talk about who would guard Hanzhong. Since Liu Bei had already let Guan Yu guard Jingzhou, and Zhang Fei's seniority and prestige in the military were second only to Guan Yu, people thought that Liu Bei would let Zhang Fei take on this heavy responsibility, and Zhang Fei also thought that it should be so. But surprisingly, Liu Bei actually chose Wei Yan to take on this important task ("Ba Yan was the governor of Hanzhong, the general of Zhenyuan, and the Taishou of Hanzhong"), and the Manchu Dynasty Wenwu was very puzzled by this.
In order to serve the people, Liu Bei promoted Wei Yan to the rank of general of Zhenyuan and asked him to express his determination in front of the courtiers. Wei Yan said loudly, "If Cao Cao comes from all over the world, please refuse for the Great King; if you will have a crowd of 100,000 people, please swallow it for the Great King." Liu Bei was very satisfied, and the Qunchen also admired Wei Yan's arrogance. Facts have proved that Wei Yan is not bragging, and his "wrong way of keeping the sieges" is very pragmatic and appropriate. "In the town of Hanzhong in Weiyan, all the soldiers were surrounded by foreign enemies, and if the enemy came to attack, they would not be allowed to enter. And the Battle of Xingshi, Wang Pinghan rejected Cao Shuang, and all inherited his system. ”
Wei Yan scattered the Shu army at every critical pass, stationing heavy troops at Luogu, Xiegu Road, Meridian Road and other places, while the rest of the soldiers and horses remained in Hanzhong City. That is to say, Wei Yan adopted a defensive strategy, arranging the defensive line on the outskirts of Hanzhong to divide the troops and hold it, forming an iron wall and copper wall, and they were each other's horns, and the Wei army could not easily break through. Later, when Wang Ping was defending Hanzhong, he followed Wei Yan's strategy and stuck to waiting for help. Therefore, Cao Shuang could not attack for a long time, so he called off his army and returned to the Dynasty. It can be said that Wei Yan has been stationed in Hanzhong for more than ten years, impregnable and unbreakable.
Later, Zhuge Liang died, and Wei Yan was taken away by hand, and the town guard Was replaced by Jiang Wei, who was most trusted by Zhuge Liang. However, in August 263, Cao Wei sent Zhong Hui, Deng Ai, and Zhuge Xu to divide into three routes and vigorously attack Shu. In less than a month, the Yang'an Pass in Hanzhong County was declared lost, and Zhong Hui led the main force of the Wei army to drive straight in. Why was it that Hanzhong, which had always been impregnable, was broken by Wei Jun in less than a month into the hands of Jiang Wei?
First, Jiang Wei changed Wei Yan's strategy. Jiang Wei believed that Wei Yan was too conservative, and although the "wrong method of defending the siege" could be defended against the enemy outside Hanzhong City, it was not conducive to the Attack of the Shu Army, let alone the enemy who attacked. Jiang Wei thought of destroying the Wei army in a war, so Jiang Wei gave up the outer barrier and distributed the army in the two cities of Han and Le, focusing on the active defensive strategy of "gathering troops in the valley, luring the enemy to go deep, clearing the wilderness with a strong wall, and annihilating the enemy", by luring the enemy to go deep and annihilating the enemy.
However, there is a premise of "Gathering Soldiers in the Valley", that is, it is necessary to defend Seoul, Lecheng, and Yang'an Pass! If you can't hold on, don't say that the total annihilation of the Wei army, Hanzhong lost only in the blink of an eye! As a result, Yang'an Guanguan surrendered Jiang Shukaicheng, causing the gateway to the Shu Road to open widely. Art of War: "Know yourself and know the other, and never lose a battle." Jiang Wei's set of "Gathering Soldiers and Gathering Valleys" is actually a typical example of not knowing oneself or knowing the other! He overestimated the combat effectiveness of the Shu army at that time, and also overestimated the loyalty of the Shu generals to the Shu Han.
Second, the imperial court of the Shu state was infighting, and the national strength was declining. During Zhuge Liang's period, because Zhuge Liang ruled the country well, the national strength of Shu Han still existed, although the Northern Expeditions were successive, the people did not complain, the officials did not talk, and Wei Yan also had the ability to defend Hanzhong and resist the enemy outside the country. However, during the Jiang Wei period, the imperial court was in power, and the imperial court did not support Jiang Wei's Northern Expedition, and in the later period, the eunuch Huang Hao and some officials, including Zhuge Liang's son Zhuge Zhan, wanted to impeach Jiang Wei, and Jiang Wei was isolated in the court and could not get Liu Chan's support, and the power struggle greatly dispersed the cohesion of Shu Han.
At this time, after the great defeat at the Battle of Liu Beiyiling, a series of military operations by Zhuge Liang's six out of Qishan, and Jiang Wei's nine expeditions to the Central Plains, the army has been depleted, but it cannot be replenished. Although Wei, Shu, and Wu barely formed the trend of three divisions under the heavens, the Three Kingdoms were nearing the end, with the State of Wei being the strongest, Eastern Wu second, and Shu Han the weakest. The loss of Hanzhong and the demise of the Shu kingdom are not Jiang Wei's pot alone, and the terminal illness is not a day's work, for many reasons, and the responsibility should not be borne by Jiang Wei alone.
(Reference: Romance of the Three Kingdoms, etc.)