Question: According to Zhou Yu, why were Cao Cao's generals unfit for warfare at the Battle of Red Cliffs?
About the article "Battle of Red Cliffs", section "Analysis".

Candidate passages:
A combination of Cao Cao's strategic errors and the effectiveness of Huang Gai's ruse had resulted in the allied victory at the Battle of Red Cliffs.
Zhou Yu had previously observed that Cao Cao's generals and soldiers comprised mostly cavalry and infantry, and few had any experience in naval warfare.
Cao Cao also had little support among the people of Jing Province, and thus lacked a secure forward base of operations .
Despite the strategic acumen Cao Cao had displayed in earlier campaigns and battles, in this case he had simply assumed that numerical superiority would eventually defeat the Sun and Liu navy.
Cao's first tactical mistake was converting his massive army of infantry and cavalry into a marine corps and navy: with only a few days of drills before the battle, Cao Cao's troops were ravaged by sea-sickness and lack of experience on water.
Tropical diseases, to which the southerners had long been immune, also plagued the soldiers of the north with the debilitating effects of sickness rampant in Cao Cao's camps.
Although numerous, Cao Cao's men were already exhausted by the unfamiliar environment and the extended southern campaign, as Zhuge Liang observed: "Even a powerful arrow at the end of its flight cannot penetrate a silk cloth" (Military Documents 1979:193).
The uncharacteristically poor preparation and miscalculations displayed by Cao Cao during this campaign may have been partly due to the recent death of his strategist and advisor Guo Jia.
Cao Cao himself had commented: "Had Guo Jia been with us, I would never have got into such trouble" .
Another key advisor, Jia Xu, had recommended after the surrender of Liu Cong that the overtaxed armies be given time to rest and replenish before engaging the armies of Sun Quan and Liu Bei, but Cao Cao disregarded the advice .
Cao Cao's own thoughts regarding his failure at Red Cliffs suggest that he held his own actions and misfortunes responsible for the defeat, rather than the strategies utilised by his enemy during the battle: "... it was only because of the sickness that I burnt my ships and retreated.
It is out of all reason for Zhou Yu to take the credit for himself."
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Zhou Yu's death is mentioned in Chapter 57 of "Sanguo Yanyi".
In the previous chapter, Zhou Yu came up with a plan to help Sun Quan seize Jing Province from Liu Bei.
He pretended that he wanted to help Liu Bei attack Yi Province (covering present-day Sichuan and Chongqing) in western China, and asked for passage through Jing Province.
When Liu Bei agreed, Zhou Yu was overjoyed because his true intention was to conquer Jing Province when his army passed through.
However, Zhuge Liang saw through Zhou Yu's ruse and set up a trap in Jing Province.
Zhou Yu fell into the trap and was surrounded by Liu Bei's forces.
He was overwhelmed by fury and fell off his horse.
Zhuge Liang later wrote a letter to Zhou Yu, asking the latter to give up on attacking Yi Province and return to Wu instead because Cao Cao might take advantage of Zhou's absence to invade Wu.
Zhou Yu then wrote to Sun Quan and told the other Wu generals to give their full support to their lord.
He fainted afterwards, regained consciousness and sighed, "If (Zhou) Yu were to be born, why must (Zhuge) Liang exist as well?"
He died after repeating that line several times.
This was the third time Zhou Yu had been infuriated by Zhuge Liang after sustaining an arrow wound at the Battle of Jiangling, a follow-up to the Battle of Red Cliffs.
The first time was when Liu Bei, acting on the advice of Zhuge Liang, seized control of many territories in Jing Province when Zhou Yu and his men were busy fighting with Cao Cao's forces for control of the province.
The second time was when Zhuge Liang foiled his "beauty scheme".
(See #Liu Bei and Lady Sun's marriage.)
Zhou Yu's condition worsened after each incident and he eventually died on the third time. 
Alliance with Sun Quan.
Battle of Red Cliffs.
When Liu Bei was still at Changban, Sun Quan's envoy Lu Su hinted to him that he should ally with Sun against Cao Cao.
Zhuge Liang went to meet Sun Quan, as Liu Bei's envoy, together with Lu Su at Chaisang to discuss the formation of the alliance.
Liu Bei and Sun Quan formed their first coalition against the southward expansion of Cao Cao.
The two sides clashed at the Red Cliffs (northwest of present day Puqi County, Hubei).
Cao Cao boasted 830,000 men (Zhou Yu claimed the realistic number be 230,000 to 270,000), while the alliance at best had 50,000 troops under the leadership of Zhou Yu.
Cao Cao's core troops were mostly northerners, so they were unable to adapt to the southern climate and naval warfare, and that posed a major disadvantage to Cao.
Subsequently, a plague broke out that undermined the strength of Cao Cao's army.
The fire attack masterminded by Zhou Yu and Huang Gai succeeded against Cao Cao's chain-linked vessels and most of Cao's navy was destroyed in the battle.
The majority of Cao Cao's troops were burnt to death or drowned in the river while the survivors who successfully retreated to the riverbank were ambushed and killed by skirmishers.
Cao Cao barely escaped after his defeat.
He retreated back to the north and left behind Cao Ren and Xu Huang to guard Jiangling and Yue Jin to defend Xiangyang. 
Sun Quan put Zhou Yu in command of his 30,000 troops, largely stationed on naval vessels.
Zhou Yu set up a defensive position in conjunction with Liu Bei, whose army garrisoned on land.
Around this time, a spreading plague significantly weakened Cao Cao's forces.
Zhou Yu's subordinate Huang Gai pretended to defect to Cao Cao's side and was accepted by the latter.
Huang Gai brought with him a small group of men and they sailed towards Cao Cao's base on boats.
As the boats approached Cao Cao's fleet, Huang Gai ordered his men to set the boats on fire, and the burning boats crashed into Cao Cao's larger ships.
The fire spread out of control and completely destroyed Cao Cao's entire naval fleet.
Cao Cao's land forces at Wulin (烏林; in present-day Honghu, Hubei) were also attacked and driven back by Sun Quan and Liu Bei's armies.
Cao Cao suffered a drastic defeat at the Battle of Red Cliffs and was forced to retreat north back to Jiangling (江陵, located in present-day Jingjiang 荆江, not to be confused with present-day Jiangling County, Hubei). 
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.