Question: What caused Cao Cao to retreat to Wulin during The Batte of Red Cliffs?
About the article "Battle of Red Cliffs", section "Battle".

Candidate passages:
The Battle of Yiling was fought between the warlords Sun Quan and Cao Cao in 208 in the late Eastern Han Dynasty.
The battle was an integral part in the Red Cliffs campaign, as it was fought immediately after the major engagement at Wulin (烏林; in present-day Honghu, Hubei) during the Battle of Red Cliffs.
The Battle of Yiling was also the prelude to the subsequent Battle of Jiangling. 
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 late 208, the northern warlord Cao Cao launched a campaign aimed at wiping out opposing forces in southern China, and successfully pressured Liu Cong (who had succeeded his father Liu Biao as the Governor of Jing Province) into surrendering.
A few months later, Cao Cao assembled a force of 800,000 at Jiangling (江陵; in present-day Jingzhou, Hubei), where countless warships and weapons were stored, and sent an envoy to intimidate Sun Quan to give up resistance.
Sun Quan eventually hardened his decision to go to war with Cao Cao after being persuaded by Lu Su and Zhou Yu, and he formed an alliance with another warlord Liu Bei.
Ling Tong, under Zhou Yu's command, participated in the Battle of Red Cliffs, in which the combined armies of Sun Quan and Liu Bei dealt Cao Cao's massive fleet a crushing defeat at Wulin (烏林). 
The Battle of Red Cliffs unfolded in three stages: an initial skirmish at Red Cliffs followed by a retreat to the Wulin (烏林) battlefields on the northwestern bank of the Yangtze, a decisive naval engagement, and Cao Cao's disastrous retreat along Huarong Road.
The combined Sun-Liu force sailed upstream from either Xiakou or Fankou to Red Cliffs, where they encountered Cao Cao's vanguard force.
Plagued by disease and low morale due to the series of forced marches they had undertaken on the prolonged southern campaign , Cao Cao's men could not gain an advantage in the small skirmish which ensued, so Cao Cao retreated to Wulin (north of the Yangtze River) and the allies pulled back to the south .
Cao Cao had chained his ships from stem to stern, possibly aiming to reduce seasickness in his navy, which comprised mostly northerners who were not used to living on ships.
Observing this, divisional commander Huang Gai sent Cao Cao a letter feigning surrender and prepared a squadron of capital ships described as "mengchong doujian" (蒙衝鬥艦).
The ships had been converted into fire ships by filling them with bundles of kindling, dry reeds, and fatty oil.
As Huang Gai's "defecting" squadron approached the midpoint of the river, the sailors applied fire to the ships before taking to small boats.
The unmanned fire ships, carried by the southeastern wind, sped towards Cao Cao's fleet and set it ablaze.
Within a short time smoke and flames stretched across the sky, and a large number of men and horses either burned to death or drowned .
Following the initial shock, Zhou Yu and the allies led a lightly armed force to capitalise on the assault.
The northern army was thrown into confusion and was utterly defeated.
Seeing the situation was hopeless, Cao Cao then issued a general order of retreat and destroyed a number of his remaining ships before withdrawing .
Cao Cao's army attempted a retreat along Huarong Road, including a long stretch passing through marshlands north of Dongting Lake.
Heavy rains had reduced the track to a thick mire, making the road so treacherous that many of the sick soldiers had to carry bundles of grass on their backs and use them to fill the road, to allow the horsemen to cross.
Many of these soldiers drowned in the mud or were trampled to death in the effort.
To the misery of Cao Cao's army, the allies, led by Zhou Yu and Liu Bei, gave chase over land and water until they reached Nan Commandery (南郡; present-day Jiangling County, Jingzhou, Hubei); combined with famine and disease, this decimated Cao Cao's remaining forces.
Cao Cao then retreated north to his home base of Ye (in present-day Handan, Hebei), leaving Cao Ren and Xu Huang to guard Jiangling, Yue Jin stationed in Xiangyang, and Man Chong in Dangyang .
The allied counterattack might have vanquished Cao Cao and his forces entirely.
However, the crossing of the Yangtze River dissolved into chaos as the allied armies converged on the riverbank and fought over the limited number of ferries.
To restore order, a detachment led by Sun Quan's general Gan Ning established a bridgehead in Yiling to the north, and only a staunch rearguard action by Cao Ren prevented further catastrophe (; ).
The Battle of Jiangling was fought by the allied forces of Sun Quan and Liu Bei against Cao Cao in the late Eastern Han Dynasty.
The battle was an integral part of the Red Cliffs campaign, and was fought immediately after the Battle of Yiling in 208, and the preceding engagement at Wulin (烏林; in present-day Honghu, Hubei) on land and the marine Battle of Red Cliffs where Cao Cao's navy was destroyed.
Note that the battle at Wulin was a byproduct of the Battle of Red Cliffs, and they were not the same battle.
While the fighting around the city of Jiangling was vigorous, there were not much fierce battles taking place in southern Jing Province.
Unable to isolate Jiangling from its supporting cities (except those in Yi Province, see Battle of Yiling (208) for details), the campaign became a war of attrition, which resulted in enormous casualties for Cao Cao's side.
After a year or so, Cao Cao could no longer afford the continuous losses in personnel and materiel, so he ordered Cao Ren to withdraw from Jiangling.