This problem should be seen from two aspects.
One is why Shu Han never forgot to cut down Wei, and the second is why Wei Guo's harassment of Shu Han can be tolerated, rather than directly sending troops to fight a decisive battle.
Let's start with the first question.
The Northern Expedition of the Shu Han Dynasty to the Central Plains was an established policy formulated twenty years ago, and it was also the consistent ideal of Zhuge Liang, the ruling faction of the Shu State.
The reason why Zhuge Liang insisted on the Northern Expedition was already clearly stated in his "Table of Later Departures": "If you do not cut down thieves, Wang Ye will also die." But sit and wait for death, and cut it down? ”
Zhuge Liang analyzed the strength of both Shu Han and Cao Wei and gave the reason for the Northern Expedition: Shu did not cut down Wei, but could only wait for death. Therefore, Zhuge Liang "bowed down to exhaustion, and died after that." As for the success or failure of the blunt, non-minister's wisdom can reverse it. ”
Shu Fa Wei, there are several reasons.
First, the Northern Expedition was Zhuge Liang's ideal
Zhuge Liang was a man greatly influenced by Confucianism, loyal to han orthodoxy, loyal to the Liu Han dynasty, and it was his ambition to maintain han orthodoxy. As early as before he left the mountain, he had determined the political ideal of being a hero like Guan Zhong and Le Yi to support the world. "Reviving the Han Room" is his life's goal.
However, Cao Cao's "holding the Heavenly Son's Order princes" was powerful; Eastern Wu had gone through three generations, and Jiangdong was attached to his heart, and he was also an object that could not be rushed.
The Shu Han regime was located in the southwest, and its strength was the worst among the three families, but Zhuge Liang did not give up his ideals, he stabbed his indomitable soul, knowing that he could not do anything, just like a moth to a fire, a hundred deaths without remorse.
Second, the Northern Expedition was a consistent strategy of the Shu Han Dynasty
When Longzhong was right, Zhuge Liang and Liu Bei formulated the strategic idea: "If there is a change in the world, then order a general to take the army of Jingzhou to Xiangwan and Luo, and the general to lead the people of Yizhou out of Qinchuan".
Out of Qin Chuan, fighting for the right of Long, occupying Guanzhong, in order to compete with Wei for the world. Because the Guanlong area was the gateway to the Shu Han Dynasty, if this area was obtained, "Wei could not cross the Sword Pavilion to collect Northern Shu, nor could it go around the steps and exchange the text for Western Shu, and Shu could survive on its own."
Shu Han only had one prefecture, and could only constantly attack and seize more territory. The Qinlong region had a large population, and Cao Wei's rule was not yet stable, and it was the base for the rise of Liu Bang, with fertile land and abundant products. Eight hundred miles of Qinchuan is rich in grain, and grain is the foundation of a country.
Shu is mountainous, and if you can seize the Qinlong region, use the resources of warhorses here to build a cavalry force, you may be able to save the situation and play an important role in reviving the Han Dynasty.
Third, it is possible to transfer the internal contradictions of the Shu and Han Dynasties.
In fact, the little Shu Han regime was full of factions and contradictions. The Jingzhou Group represented by Liu Bei, the Dongzhou Group represented by Li Yan, and the Yizhou Group represented by Fazheng are three interest groups that are inclined to each other and have many contradictions.
Especially after Liu Bei's death, there was no one who could suppress the three groups, how did Zhuge Liang balance the interests of the three groups?
Only by adopting the technique of the law, ruling the country with severe punishment, and taking the initiative to attack the outside world can we use the laws of wartime to carry out governance, use wartime law to control the interest groups that arouse interests at home, transfer domestic contradictions to external contradictions, and alleviate domestic contradictions.
The fourth advocates the orthodox status of the Shu Han regime.
The Shu Han regime has always regarded itself as the orthodox Han Dynasty, and it is the task and responsibility of the Shu Han regime to eliminate Cao Wei and restore the Han Dynasty. If you are in a corner of peace and do not want to make progress, how can you talk about orthodoxy? Doing so is bound to be a loss of popular will, so why should we gain a foothold?
Shu Han was a small country with few talents. How to discover talents, find talents, and serve the Shu Han regime?
We know that the military is a melting pot, and war is the most tempering. Only through war, discovering and cultivating talents can the transfer of power be realized.
This is also the reason why Zhuge Liang, after discovering Jiang Wei, cultivated Jiang Wei as a successor and taught jiang Wei his lifelong skills.
Through the war, Zhuge Liang also discovered Generals such as Jiang Huan, Wang Ping, Zhang Ning, and Wu Ban, and discovered new forces for shu Han.
If Shu Han sat on Xichuan, the gap between its strength and that of Cao Wei would only grow. Whether it was population, land, or financial resources, the State of Wei was much more powerful than the Shu Han. So "Sit and wait for death, or cut it down?" "If annihilation is inevitable, then why not fight?"
Let's talk about the second question, look at the Wei side.
Since the ancient Shu Dao was difficult to go to the Qingtian, it was Shu that had natural dangers, so attacking Shu was a thankless task.
As early as the time of King Hui of Qin, Zhang Yi and Sima Que had argued over whether to cut down Shu. Zhang Yi believed that Shuzhong was remote and dangerous, and if it was attacked hastily, even if it was fought, it would make the soldiers tired, and the burden of the common people would increase, even if the Shu land was captured, it would not be worth the loss. It is a loss-making transaction that "the evil soldiers and the masses are not enough to become famous, and the land is not enough for profit".
(Sima Yi stills)
As an enemy of Cao Wei, Zhuge Liang's worries were what Wei had hoped. Because if the two sides were allowed to develop, it would only make the gap in national strength wider and wider, and the day shu Han sat and waited to die was when Cao Wei's soldiers captured Xichuan without bloodshed.
For Cao Wei, the biggest enemy was not Shu Han, but the State of Wu. Wei had taken the initiative to launch three major wars with Wu, indicating that in the eyes of the State of Wei, the threat of Eastern Wu was much greater than that of Shu Han.
In fact, the State of Wei also took the initiative to cut down Shu. In the fourth year of Wei Taihe, cao zhen was sent as a general, and he was cut down on the grounds that Shu Company had sent troops to invade the territory. However, the effect of finding Shu this time was not good, because the Wei army also faced the same problem as the Northern Expedition of the Shu army: the problem of grain and grass.
The plan of the Northern Expedition was to "send Chang'an in August and enter south from the Meridian Road." King Sima Xuan traced the Han River and met Nan Zheng". However, the ending is "there will be a rain of more than thirty days, or the boardwalk will be cut off, and the zhaozhen will return to the army."
It was precisely because of transportation and logistical problems that many of the Wei ministers were opposed to the shu.
The Book of Wei - Biography of Liu Ye records that "the emperor will cut down Shu, and the courtiers inside and outside will not be allowed."
After this temptation, Cao Rui also knew that Shu shu should not be rushed to make quick gains, and he took Hua Xin's advice, admitting that Shu Han could not easily conquer even his grandfather and father. ”。
Therefore, Cao Wei's policy towards Shu was defensive, and Cao Rui repeatedly asked Sima Yi to resist the wall, and Sima Yi knew its magic, so he was not moved by the women's clothes sent by Zhuge Liang.
There is also a saying that the Sima family raised Kou self-respect. It was to keep Shu Han as a chip for the Wei state to need Sima Shi. But there is no basis for this claim, and credibility needs to be questioned.
As for Cao Wei, the best thing was that Wei and Shu were at peace with each other, and Wei guo could sit back and wait for the strength of the two countries to grow day by day, and finally make a big effort to sweep away Xichuan with a thunderous momentum.
This was the most correct and clever choice for Cao Wei.
Only when Wei Ganlu was three years old and Sima Zhao was preparing to unify the world, at this time the power of the Shu state had declined, the talent had withered, and the strength of the two sides was no longer in the same order of magnitude, that Zhong Hui and Deng Ai were able to take Shu like a bamboo.