In the "Romance of the Three Kingdoms", Cao Cao lost the most tragic battle of Chibi, before the war, Cao Cao also wrote a letter to Sun Quan, wanting to gain a psychological advantage. Later, after Zhou Yu went to Fanyang, he also learned about this matter, and he thought that Cao Cao was a traitor and that Er and others should lead troops to fight him.
Moreover, most of Cao Cao's soldiers were central plains people and were not good at water warfare, and Ma Chao and Han Sui were still stationed at HanguGuan staring at Cao Cao's "fat meat".
When is it better not to fight at this time? Later, Sun Quan united with Liu Bei and planned to attack Cao Cao together.

Fire to the ship
Cao Cao also knew that his soldiers were all northerners, did not like to take ships, and specially united the heads and tails of each ship to prevent the soldiers from getting seasick, but he would never have thought that it was his decision that had harmed the entire Cao army. At that time, Huang Gai took more than a dozen ships filled with oil and firewood to surrender to Cao Cao.
When approaching Cao Ying, they lit firewood, and more than a dozen ships carrying dry firewood and fire oil rushed to Cao Ying
Cao Cao's ship was burned by the sudden fire, and Cao Cao himself fled in a daze. Zhuge Liang also calculated that Cao Cao must have fled in the direction of Huarong Dao, so he sent Guan Yu to intercept it. But when Cao Cao faced Guan Yu, he not only did not have fear, but also laughed three times.
Cao Cao smiled three times
The first time Cao Cao laughed at Zhao Yun's incompetence and failed to block Xu Huang and Zhang Guo, and the second time he laughed at Zhang Fei for not being able to block Xu Huang and Zhang Gao's attacks. Then this third laugh is very meaningful. This laugh directly made Guan Yu let Cao Cao go back in order to remember Cao Cao's benevolence to himself in Cao Ying before.
Many people here expressed regret, if Guan Yu killed Cao Cao at this moment, maybe the final winner would be Liu Bei
。 So is this really the case? No, if Cao Cao was killed, it would not be Liu Bei but Sun Quan who would unify the three kingdoms.
Take advantage of this to seek revenge
If Cao Cao died in battle at Huarong Dao, then the situation in the north that he had just calmed would be reversed in an instant. His sons Cao Pi, Cao Zhang, and Cao Zhi were bound to fight for power, and there would be major turmoil within the entire Cao Army, which was quite unfavorable to the Cao Army, which had just stabilized the situation.
Moreover, in their rear, Ma Chao and Han Sui were still staring deadly, perhaps waiting for the civil unrest in the Cao Army to take advantage of the situation.
There were also some nomadic peoples in the north, cao cao had killed their leaders before, if those people knew, they might take this opportunity to seek revenge, and then Cao Wei would be in danger of being attacked in the belly.
Decapitated
However, Cao Cao's biggest hidden danger was not Ma Chao or Shu Han, but Sun Quan. Once Cao Cao died, the Cao army was leaderless, and Sun Quan was the strongest at that time, and he would definitely find a way to annex Cao Jun, and then take Liu Bei down, and once Liu Bei died, Sun Quan could take Jingzhou, a treasure of feng shui, in his hands. At that time, no matter how arrogant Ma Chao and others were, it would not be enough to be afraid. Therefore, Zhuge Liang deliberately put Guan Yu in Huarong Dao, because only Guan Yu had an old relationship with Cao Cao, and if he had changed to Zhao Yun or Zhang Fei, Cao Cao would have died a long time ago.
After this arrangement, Sun Quan could not unify the three kingdoms, nor did he have the opportunity to wreak havoc in the Central Plains.
bibliography:
Romance of the Three Kingdoms