First of all, within the Cao Cao Group, before the Battle of Hanzhong, the contradiction between Cao Cao and Emperor Han Xian had been completely intensified, and we can see two points:
First, Cao Cao went to see Xian Di in the temple, and Emperor Xian of Han said to Cao Cao:? If you can help me, be generous; Otherwise, be merciful and put me aside. ? Cao Cao was frightened to disgrace and quickly responded to the request to leave. According to the old system of the Han dynasty, when the three fierce generals met the emperor, they had to be held hostage by a samurai with a knife. When Cao Cao came out of the temple, he looked back, sweating profusely, and never appeared in front of Xian Di. ?
Second, according to historical records: Ji Ping, a doctor too much in Han Dynasty, conspired with Ji Geng and Wei Huang of Shaofu to attack Xudu and burn down the barracks of Prime Minister Wang Bi. ? In other words, Jiping people contacted more than 1000 people in Xuchang, and attacked Cao Cao's highest military and political chief who stayed in Xuchang. The point is, he almost killed Cao Cao, the highest military and political chief who stayed in Xuchang.
Although these two things were properly solved by Cao Cao, it can be seen from these two things themselves that various forces dissatisfied with Cao Cao in the Cao Cao Group at that time were already ready to make a final confrontation with Cao Cao. In this context, who can guarantee what will happen when Cao Cao leads the main military forces away from Xuchang for a long time?
Secondly, before the fall of Hanzhong, it was precisely when Cao Cao achieved unprecedented success in Xiongnu and Wuhuan.
In this war, Cao Zhang began to shine brilliantly on the military and political stage, and the glory and glory of Cao Zhang's life will soon be obtained from Wu Huan. In World War I, Cao Zhang beat the Wuhuan people to death, and the Xianbei people on the sidelines surrendered directly in fear.
In this sense, when Cao Cao and Liu Bei competed for Hanzhong, a considerable part of Cao Cao's troops were dispersed to the north to fight against Xiongnu and Wuhuan. Because to ensure the victory of the war with the northern nomadic people, it is naturally impossible for Cao Cao to shift all his focus to Hanzhong to fight Liu Bei.
Third, Cao Cao also needs to face the threat of Sun Quan. He thought that when Cao Cao was conquering Zhang Lu in Hanzhong on a large scale, Sun Quan had seized this opportunity to attack Huainan on a large scale. Just because of coincidence, Zhang Liao was defeated by Sun Quan on the battlefield in Hefei. But the problem is that Zhang Liao's success is very lucky, because from the comparison of military strength at that time, it is estimated that no one dares to say that Zhang Liao can defeat Sun Quan.
In fact, when Sun Quan attacked on a large scale, the whole city of Hefei was in a panic. In this context, if Cao Cao and Liu Bei fight for Hanzhong with all their strength, and Sun Quan seizes the opportunity to attack Huainan on a large scale again, Cao Cao will certainly have no way to help in time. Because the two battlefields are too far apart. As for whether the battle of xiaoyaojin can happen again, Cao Cao obviously can't guarantee it, because the probability of such a thing happening is too low.
Of course, in addition to the above three threats, Cao Cao has another threat, and that is Guan Yu. Although Guan Yu's own threat is not great, the problem is that if Cao Cao put all the military and political forces into the Hanzhong War, then Sun Quan will seize the opportunity to attack Huainan on a large scale. At this time, various forces within Cao Cao Group were dissatisfied with Cao Cao and took the opportunity to launch a rebellion in Xuchang. Then Guan Yu sent troops to sweep the central plains, and there was obviously a chance.
If this is the result, the nomadic people in the north may also take the opportunity to bite back, so that the regime painstakingly managed by Cao Cao is likely to collapse in an instant. Because of this, when Liu Bei had mobilized all his manpower, material resources and financial resources to fight in the front line of Hanzhong, Cao Cao only led the army to Chang 'an and did not immediately gather in the battlefield of Hanzhong.
Because Cao Cao is located in Chang 'an, he can take care of southwest, Huainan, Central Plains and northwest.
At that time, Huainan area could be attacked by Sun Quan at any time. The central plains region may be threatened by Guan Yu and villains within the group at any time; Northwest China may be attacked by nomadic people at any time. Therefore, after Cao Cao's army arrived in Chang 'an, they dared not go west again.
In fact, when Cao Cao led the army to Chang 'an, there was a rebellion in Wancheng.
Although the rebellion from Wancheng was quickly suppressed, Cao Cao dared not easily put his main force into the Hanzhong War. In this context, Xia's pressure on the battlefield in Hanzhong is naturally increasing. Because he is equivalent to using a partial division to deal with all the main forces of Liu Bei. So Xia was beheaded by Huang Zhong and Hanzhong fell.
After learning that Hanzhong was lost, Cao Cao finally couldn't sit still and quickly led the army back to Hanzhong from Chang 'an. However, there is no turning back. Because Hanzhong has been controlled by Liu Bei, Cao Cao can be consumed to death.
In fact, Liu Bei's attitude at that time was the same as Sima Yi's attitude towards Zhuge Liang later. In a word, I have consumed you to death.
Therefore, in the subsequent war, Cao Cao became more and more unfavorable on the battlefield in Hanzhong. Because Cao Cao's logistics needs to be supplemented across the Qinling Mountains, the logistics pressure is getting bigger and bigger, and other threats are beginning to stir, especially Guan Yu in the Central Plains. In this context, Cao Cao knew that if he continued to consume, the consequences would be very dangerous, so he had to retreat.