Why was the powerful Cao Cao defeated by Liu Bei, who had just risen, in the battle of Hanzhong?

In history, in fact, Liu Bei did not have such a strong national strength, nor did he have a war history called a miracle. His victory was mostly due to his uncle's identity to attract some heroes in the Jianghu, and Zhuge Liang helped him with his dark personality and joined hands with Wu Dong, making Cao Cao, a powerful military, unable to help two small countries in the south. However, the battle in Hanzhong was purely because Cao Cao looked down on Liu Bei and didn't invest too much troops to fight with Liu Bei at all. So after the victory of Dongwu, Liu Bei's Cao Jun also retreated, which made Liu Bei win the war.

In Cao Cao's view, the biggest threat to the south is Wu Dong, because Cao Cao suffered a lot from Zhou Yu and Sun Quan before, and he naturally had some fears about Sun Quan. So he always put the military center of gravity of Cao Jun on Sun Quan's side. Take no precautions against Liu Bei. In his view, a rabble, Liu Bei, who doesn't know if he is a royal family, can defeat the Cao family with a million troops there.

For Liu Bei, it is true that all his possessions are not enough for Cao Cao to see, and he may not be as good as Sun Quan, and he can use the terrain to face Cao Cao, but at the same time there is still a big gap between Cao Cao's army and Liu Bei's army in the plain. Although Liu Bei occupied a vast area of Yizhou, some local gentry simply did not want to accept his management and rule because of their improper origins.

In the battle of Hanzhong, Liu Bei suspended the battle of Jingzhou in order to relieve Sun Quan's military pressure on the eastern front, and finally sent troops to attack Hanzhong. Cao Cao did not immediately transfer troops from ruxu to support Hanzhong, but just appropriately increased some troops and strengthened defense. After all, I didn't beat Liu Bei even if I didn't guard him.