1. Repay the late emperor and fulfill his last wish (taking Zhuge Liang as an example)
Zhuge Liang said in the "Teacher's Watch": "The late emperor knew that I was cautious, so I was on the verge of collapse. Since I was appointed, I have been worried all night, afraid that my entrustment will be invalid, which will hurt my late emperor's knowledge ... Now that the south has been decided, there are enough soldiers. The three armies gave it to me, and I was originally scheduled to go north, so I was tired and dull. Besides raping and killing, I was busy restoring the Han Dynasty and the old capital. "
It can be seen that Zhuge Liang's northern expedition was to repay Liu Bei's kindness to him and fulfill his last wish of recovering the Han Dynasty.
2. If you don't attack Cao Wei, Shu Han will die (according to Yi Zhongtian's Three Kingdoms)
Because Cao Rui, the emperor of Cao Wei, agreed to the advice of the minister's grandson, he took a firm position in Shu and vigorously restored the strength of Wei. Zhuge Liang has realized that if Cao Wei is given time to develop his national strength, the gap between him and Wei will become wider and wider.
Once Cao Wei's national strength is strong, Shu can only do nothing. Only by going out of Shu and occupying Yongzhou and Liangzhou can we get Qinchuan horses, organize cavalry, and have circuitous space to launch large-scale operations. Third, it can greatly weaken the strength of Cao Wei cavalry.
3. Contradictions between the Northern Expedition of the Central Plains and the Transfer (Basis: Yi Zhongtian Pinsan Kingdom)
Since there were Jingzhou and Yizhou in Shu at the beginning of the founding of the People's Republic of China, the contradiction between the three forces was not obvious. But since Guan Yu lost Jingzhou and Liu Bei lost Yiling.
Jingzhou Group suffered heavy casualties after losing Jingzhou and Yiling (Guan Yu, Zhang Fei, Huang Zhong, Ma Liang, Mi Zhu, etc. ), and unable to continue to suppress the powerful Shudi Group, so it is necessary to unite the Eastern Zhou Group.
Since then, contradictions have become increasingly prominent. Liu Bei also realized this, so when entrusting orphans, he appointed Zhuge Liang as "official minister" and Li Yan as "deputy minister".
On the one hand, we can let the two sides supervise each other, on the other hand, we can take the opportunity to win over Dongzhou Group. Besides, Li Yan is half of Jingzhou Group, which does not conflict with the basic national policy formulated by Liu Bei.
Zhuge Liang launched the Northern Expedition, which could transfer the internal contradictions of Shu to the external contradictions of Cao Wei. Once Yongzhou and Liangzhou are laid, the contradiction between Jingzhou, Dongzhou and Shu will be greatly reduced. And it can give Shu a chance to win the Central Plains.
Extended data:
In the fourth year of Shu Jianxing (226), Cao Pi, the Emperor of Wei Wen, died, and his son Cao Cao succeeded to the throne, lacking ruling experience. Zhuge Liang seized the favorable opportunity and decided to start the Northern Expedition. In March of the following year, he led the army to Hanzhong and sent troops to Mianyang (now Mianxian County, Shaanxi Province).
In the spring of the 6th year of lite (228), Zhuge Liang raised his voice ahead of time and took the Xiegu Road for reward, so that Zhao Yun and Deng Zhi set up suspected soldiers to attract Cao Zhen's heavy troops and led the army to attack Qishan. Nan 'an, Tianshui and Anding counties in Longyou opposed Wei's attachment to Shu, and Guanzhong shook. Chang 'an, the west town of Wei Mingdi, ordered Zhang He to ride fifty thousand troops and beat Ma Su to the street pavilion.
At the same time, Zhao Yun was outnumbered and lost to Ji Gu. Zhuge Liang pulled more than 1000 families back to Hanzhong from Xixian County. The first Northern Expedition failed. ?
In the winter of the same year, Zhuge Liang took advantage of the weakness of Guanzhong by taking advantage of the opportunity of Taiwei's eastward advance. He took advantage of the Northern Expedition to send troops to disperse customs (southwest of Baoji City, Shaanxi Province) and surround Chencang (east of Baoji City, Shaanxi Province). Zhao Hao, the general of Wei, refused him, but Zhuge Liang failed to persuade him to surrender. He had no choice but to return to Hanzhong. Wei Zhui was beheaded. ?
In the spring of the seventh year of lite (229), Zhuge Liang sent Chen Shi to attack Wudu (now Chengxian County, Gansu Province) and Yin Ping (now Wenxian County, Gansu Province). Guo Huai, the secretariat of Yongzhou, led the troops to rescue him. Out to Jianwei (now west of Xihe County, Gansu Province), Guo Huai withdrew, so he got the second county. ?
In the autumn of the eighth year of lite (230), Wei Jun attacked Hanzhong, Sima Yi went to Xicheng (now northwest of Ankang City, Shaanxi Province), Zhang He went to Woods Valley, and Cao Zhen went to Gu Jie.
Zhuge Liang was stationed in Chenggu (now east of Chenggu County, Shaanxi Province) and Akasaka (now twenty miles east of Yangxian County, Shaanxi Province). It rained heavily for more than 30 days, and Wei Jun retreated. In the same year, Zhuge Liang made Wei Yan and Wu enter Qiang, defeated Wei Hou General Fei Yao and Yongzhou secretariat Guo Huai in Yangxi.
In February of the 9th year of Jianxing (23 1), Zhuge Liang led an army to attack Qishan, only to get lucky. When Cao Zhen was seriously ill, the Guanzhong commander Sima Yi refused. Zhuge Liang cut wheat in Shangguan (now Tianshui County, Gansu Province). Sima Yi chased the light to Lucheng and dug a camp for self-defense, which was ridiculed as "fearing Shu like a tiger".
In the 10th year of Jianxing (232), Zhuge Liang persuaded farmers to stop fighting in Huangsha, made wooden cattle and horses, practiced martial arts and prepared for another northern expedition.
In the winter of the 11th year of Jianxing (233), Zhuge Liang once again sent troops to transport rice to the north of Qishan, gathered troops at Xiegukou and ruled the Xieguting. ?
In February (234) of the 12th year of Jianxing, after three years of preparation, Zhuge Liang persuaded the peasants to speak martial arts and led the army out of Gu Jie Road. According to Wu Wuzhangyuan (now south of Qishan, Shaanxi Province), he settled in Weibin and sent an envoy to Soochow, hoping that Sun Quan could attack Wei at the same time.
In April, the Shu army arrived in Yan county and camped under Wuzhangyuan on the south bank of Wei River. Sima Yi led a camp in last stand, Wei Jun, hoping to consume the food and grass of the Shu army again in the protracted war and make the Shu army retreat on its own. ?
Guo Huai, the general of Wei, thought that the Shu army would fight for the North Plain. Sima Yi agreed, so he sent Guo Huaixian to guard against it. Sure enough, the Shu army arrived, but was repelled by Guo Huai. ?
Zhuge Liang considered that the previous Northern Expeditions were unsuccessful because there was no grain transportation, so he began to reclaim land and grow grain among residents in Weibin, and the people lived in peace and everything was as usual. Sun Quan also responded to the call of Shu Han, led hundreds of troops to attack Wei, but was defeated by Wei Mingdi Cao Cao himself.
Zhuge Liang sent Meng Yan to the north of Wugong Shuibei, which coincided with the rising water and blocked the connection between Zhuge Liang and Meng Yan. Sima Yi took the opportunity to send troops to attack Meng Yan. So Zhuge Liang sent engineers to build bridges, and crossbowmen shot arrows at Sima Yi's troops.
Seeing that the bridge was about to be built, Sima Yi had to retreat. Wei and Shu armies fought for more than a hundred days, during which Zhuge Liang sent people to challenge them many times, but Sima Yijun couldn't hold on. Later, Zhuge Liang specially asked someone to bring Sima Yi a set of women's clothes and headscarves, indicating that Sima Yi was just like a woman.
General Wei Jun was furious and demanded war. In order to prevaricate the generals' demands, Sima Yi pretended to give Wei Mingdi a conclusive report. Cao Rui sent Wei Yan and Xin Pi as military advisers to control Sima Yi at the front.
Zhuge Liang is an adventurous wasteland-digger, who grows grain and is self-sufficient with the people of Wei in the territory of Wei. He planned to stay there for a long time, but Zhuge Liang was seriously ill because of overwork. ?
In August, Zhuge Liang really fell ill because of overwork, and his condition deteriorated day by day. Sima Yi took advantage of Zhuge Liang's serious illness and failed to unify the army. He personally led the army to attack the rear of the Shu army, beheaded more than 500 people, seized more than 1,000 livestock and left more than 600 people behind.
When the news that Zhuge Liang was seriously ill reached Chengdu, Liu Chan sent Li Fu to see Zhuge Liang and asked about the country's future plans. Zhuge Liang also told people that Yang Yi and Fei Yi led the troops to retreat.
Wei Yan and Jiang Wei are in charge of patrol. Soon, Zhuge Liang died in the military camp. According to Zhuge Liang's final deployment, Yang Yi and Jiang Wei kept a low profile, did not send out obituaries, reorganized the army and retreated calmly.
Baidu encyclopedia-Zhuge Liang's northern expedition