Article: Battle of Red Cliffs
Section: Battle

Question: What caused Cao Cao to retreat to Wulin during The Batte of Red Cliffs?

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 (; ).