Question: Why did Cao Cao fall back in the Battle of Red Cliffs?
About the article "Battle of Red Cliffs", section "Battle".

Candidate passages:
  December 10 – Cao Cao writes "Duǎn Ge Xíng".
Cao Cao marches south with his army and captures the enemy fleet at Jiangling.
Liu Bei escapes from Cao Cao at the Battle of Changban.
Zhou Yu and Liu Bei defeat Cao Cao at the Battle of Red Cliffs; along with the Battle of Yamen and Battle of Lake Poyang, this is one of the largest naval battles in China's history.  
This incident is not mentioned in the "Sanguozhi" and is entirely fiction.
The "Shanyang Gong Zaiji" (山陽公載記) stated that after Cao Cao lost the Battle of Red Cliffs, he retreated with his surviving men and passed by Huarong Trail.
The path was muddy and difficult to access, so Cao Cao ordered the weaker soldiers to lay the ground with straw and hay so that his horsemen can pass.
Many of those weaker soldiers were trampled to death when they became stuck in the mud.
When Cao Cao finally got out of the dire situation, he expressed joy so his generals asked him why he was happy.
Cao Cao replied, "Liu Bei, he's my mate.
However, he doesn't think fast; if he had set fire earlier I'd have no chance of escaping."
Liu Bei did think of setting fire but it was too late as Cao Cao had already escaped. 
In Luo Guanzhong's historical novel "Romance of the Three Kingdoms", Cheng Yu offered a ruse in order to get Xu Shu to serve Cao Cao.
At the time Xu Shu was serving as rival Liu Bei's key strategist and managed to score a major victory against Cao's generals Lü Kuang (呂嚝), Lü Xiang (呂翔), and Cao Ren (曹仁).
Exploiting the fact that Xu Shu was an extremely filial person, Cheng Yu suggested to Cao Cao to hold her mother hostage and force Xu Shu to leave Liu Bei and serve Cao.
Cheng Yu wrote a fake letter to Xu Shu and successfully tricked Xu to come to Xuchang.
Ironically, Xu Shu's mother committed suicide after seeing her son fall for such a ruse and leaving a righteous person like Liu Bei to serve under the ruthless Cao Cao.
Prior to the Battle of Red Cliffs, Cheng Yu had predicted that Sun Quan's forces would use fire to attack Cao Cao's naval fleet.
However, Cao Cao did not heed his advice seriously as the winds were to their advantage then.
After Cao Cao's major defeat, Cheng Yu was one of the few advisors who stood by Cao Cao all the way during their escape. 
In 199, Yuan Shao, a major warlord at that time, sent a request for Zhang Xiu to join him in his fight against Cao Cao.
Zhang Xiu wanted to accept the request; however, Jia Xu turned away the messenger and told Zhang Xiu that Yuan Shao was not worth serving.
Ultimately, Jia Xu advised Zhang Xiu to surrender to Cao Cao, a surrender which was well received by Cao.
Jia Xu then became an advisor to Cao Cao and later Cao Pi.
Jia Xu helped Cao Cao defeat Yuan Shao in the Battle of Guandu by advising Cao Cao to cut off Yuan Shao's grain supply, which succeeded and became an important factor of the decisive victory of Cao Cao, and eventually his unification of northern China.
In 208, after Cao Cao took Jing Province from Liu Biao, Jia Xu suggested Cao Cao not to attack the southeastern warlord Sun Quan, an advice which was rejected, and Cao Cao soon regretted the decision when his entire army was demolished at the Battle of Red Cliffs.
Soon afterwards, Cao Cao was attacked by an army led by Han Sui and Ma Chao in the Battle of Tong Pass.
Jia Xu laid out a plan which turned Han Sui against Ma Chao and ensured victory for Cao Cao.
Jia Xu was a major supporter of Cao Pi, Cao Cao's eldest surviving son, in his conflict with his younger brother Cao Zhi for the succession of Cao Cao, and successfully talked Cao Cao into selecting Cao Pi. 
Guan Yu was sent to guard Huarong Trail (believed to be near Dongshan Town, Huarong County, Hunan) and intercept Cao Cao when the latter was retreating after his defeat at the Battle of Red Cliffs.
Initially, Zhuge Liang was reluctant in allowing Guan Yu to take up that responsibility, since he was worried that Guan might take into account the kindness Cao Cao showed towards him when he temporarily served Cao years ago, and release Cao as a result.
Guan Yu insisted, claiming that he had already repaid Cao Cao's kindness by slaying Yuan Shao's generals Yan Liang and Wen Chou and lifting the siege on Boma.
He made a military pledge not to let Cao Cao off, while Zhuge Liang also stated that he would give up his head to Guan Yu if Cao Cao did not pass through Huarong Trail.
As Zhuge Liang predicted, Cao Cao did reach Huarong Trail and encountered Guan Yu there.
However, Guan Yu decided to spare Cao Cao in consideration of their past dealings, and after taking pity on the plight of Cao Cao's men and Zhang Liao, whom he saved from death earlier after the Battle of Xiapi.
He allowed Cao Cao and his men to pass through Huarong Trail unharmed.
When he returned to camp he confessed the truth to Liu Bei and Zhuge Liang.
Zhuge Liang ordered Guan Yu to be executed for failing in his mission and agreed to pardon Guan when Liu Bei intervened.