A few days ago, he wrote an article titled "Wei Yan resisted Cao Wei in Hanzhong for 40 years, why did he lose it in a month in the hands of Jiang Wei?" Of course, this article question is wrong (others asked), Jiang Wei Zhen Shou Hanzhong is definitely not a month, but 25 years, so in this answer, the focus of my answer is the strategy that Wei Yan adopted when he was in Zhen Shou Hanzhong.
In that article, I mainly distinguished the historical Wei Yan from the literary image of Wei Yan, and in this article, I also hope to use Jiang Wei's Northern Expedition to show the situation of the Shu State after Zhuge Hou's death.
In the literary image, Jiang Wei is a successor to Zhuge Liang's true biography, and the Shu kingdom can be sustained to the end by Jiang Wei in the post-Zhuge Liang era, many historians do not think so, most people think that it is actually Jiang Wei who is poor in military force, and the repeated Northern Expeditions eventually led to the demise of the Shu state.
The peak period of the Shu state was when Liu Bei captured Guanzhong, Guan Yu had not yet lost Jingzhou, if the Shu state could seize the opportunity at that time, it was not impossible to complete Zhuge Liang's so-called Longzhong pair in the future, but after Guan Yu lost Jingzhou, Liu Bei lost to Eastern Wu at Yiling, and after these two heavy hammers, Shu Han could be said to be seriously injured.
If no attempt was made to change this political pattern, the Shu state would certainly have no future, but Zhuge Liang's many northern expeditions did not achieve great success, only the first northern expedition gave the Cao Wei regime a great deterrent, but because of the employment problem, it eventually led to the failure of the northern expedition.
Zhuge Liang's Northern Expedition lasted for many years, completely hollowing out the national strength of the Shu state, and after Zhuge Liang's death, Jiang Wei took the lead in presiding over the two Northern Expeditions.
The first Northern Expedition was in 238 AD, when Jiang Wei "led his division west to take Long Right" and fought fiercely with the Wei general Guo Huai, with the result that "Wei and Wei generals rejected Nan'an", and this battle was considered to be a draw;
The second Northern Expedition was in 244 AD, when Jiang Weibu attacked Cao Shuang with Wang Ping, and Jiang Weichi aided Wang Ping, defeating Cao Shuang and winning a victory.
Both Jiang Huan and Fei Yi advocated recuperation, only Jiang Wei was the main battle, this should be the time when the Cao Wei regime was unstable, and it was also the best opportunity for the Shu state, but due to the obstruction of Jiang Huan and Fei Yi, the scale of the Northern Expedition was limited, and only once achieved a slight victory, and the other achievements were basically drawn with the Wei state.
The Third Northern Expedition was in 248 AD, when Jiang Wei led an army out of Longxi, Annan, and Jincheng to fight against the Wei state guo huai and Xiahou Ba, but failed to achieve substantial results;
The Fourth Northern Expedition was in 249 BC, when Jiang Wei sent Liao Hua as a forward to attack Taocheng, but after encountering the Wei general Deng Ai, he did not return.
The fifth time, in 254 AD, Jiang Wei returned to Xiping and met Guo Huai again in Taoxi, still without returning.
After the deaths of Jiang Huan and Fei Yi, Jiang Wei gradually took charge of the Shu Han regime as a disciple of Zhuge Liang, presided over six Northern Expeditions, and fought in the dark, at this stage, the Cao Wei regime was already stable, not so easy to fight.
The Sixth Northern Expedition, in 254 AD, this Northern Expedition was very large, leading tens of thousands of Shu troops in the Northern Expedition, and as a result, they ran out of grain and retreated.
In the Seventh Northern Expedition, in 255 AD, Jiang Wei set out from Di Dao in Longxi, entered Xiangwu, confronted the Wei general Xu Qian, beheaded and broke the enemy", and took advantage of the victory to "return the people of the three counties of Heguan, Didao, and Lintao".
During the Eighth Northern Expedition, in 256 AD, Jiang Wei was still out of Di Dao, defeating the Wei generals again in Taoxi, and finally retreating.
In the Ninth Northern Expedition, in 257 AD, Jiang Wei and Deng Ai fought a major battle at Duangu, but because the reinforcements did not arrive, Jiang Wei was defeated by Deng Ai, and the Shu state suffered heavy casualties.
In the tenth Northern Expedition, in 258 AD, the Wei general Zhuge Wei rebelled against Wei, and Jiang Wei immediately led his troops to respond, but Zhuge Zhao's army was defeated, and Jiang Wei could only retreat.
The eleventh Northern Expedition was in 262 AD, when Jiang Wei went to Qinchuan and was defeated by Deng Ai.
In the eleven Northern Expeditions that Jiang Wei presided over or participated in before and after, Jiang Wei won three games, lost two games, and drew six games, which was quite OK overall. However, this record is that most of the victories are in the front, and the defeats are in the back, that is to say, the scale of the early battles is not very large, and the warring states that can be achieved are also limited, and the ninth and eleventh defeats by Deng Ai can be said to make Deng Ai feel good.
More importantly, this kind of war was still very much pressure on the economic pressure of the Shu state, and the economy of the southwest region was not developed during the two Han and three kingdoms period, whether it was territory or population, the Shu Han did not have the qualifications to spend with the Wei state, and the final result was that the Shu state was getting poorer and poorer.
After having an overall understanding of Jiang Wei's Northern Expedition, let's look at where Jiang Wei's Northern Expedition hit the farthest point.
In the course of the Northern Expedition, the farthest place was near Didao and Lintao, probably near lanzhou in gansu.
If you just look at the flat map, everyone may be a little confused, the Northern Expedition of the Shu Kingdom did not fight in the northeast direction, but from the northwest direction? This is the geographical location of the decision, the Shu state wants to northern expedition must take Guanzhong, but the face from the Qinling Mountains is very small, Wei Yan's Meridian Valley plot is to pass through the Qinling Mountains, directly to Guanzhong, because the danger coefficient is too high.
Therefore, from the time of Zhuge Liang, the Northern Expedition strategy of opening up a circuitous northwest and then advancing into Guanzhong was relatively gentle and likely to be supported by local ethnic minorities, which gave the Cao Wei regime a great deterrent during Zhuge Liang's first Northern Expedition, but after that, Cao Wei took this line as an important route to guard against Shu Han.
This Northern Expedition route needed more grain because it was far away, and whether it was Zhuge Liang's Northern Expedition or Jiang Wei's Northern Expedition, it was often not fought, and there was no grain first.
From the fact that Jiang Wei was able to fight to the farthest place, it can be seen that Jiang Wei's Northern Expedition strategy did not actually break through the core problems of Zhuge Liang's Northern Expedition, and the final result was similar.