Battle of Chibi (Red Cliff)After the defeat of Yuan Shao and his victorious campaign against the Wuhuan, Cao Cao more or less controlled the northern plain. In the seventh month of 208, he moved his armies south via Wan, with the aim of swallowing Liu Biao and destroying Sun Quan, to cement his unification of China. In the ninth month, Cao's forces reached the outskirts of Xinye. By this time Liu Biao had already died and his successor Liu Zong surrendered without a fight. Nevertheless, Liu Biao's subordinate Liu Bei refused to submit and retreated south with his troops from Fan. After absorbing Liu Biao's forces, Cao Cao caught up with Liu Bei and routed him at Changban. Liu Bei and his retinue managed to escape and met up with his commander Guan Yu at Xiakou. Immediately, Liu sent his advisor Zhuge Liang to Chaisang to discuss a pact against the impending invasion with Sun Quan. Such a move was widely supported by the war faction on Sun's side, which included Lu Su and Zhou Yu.
After the decision was made to fight, Sun Quan marshalled his armies and made Zhou Yu commander-in-chief and Cheng Pu second-in-command. Cao Cao had boasted that his army number 800,000. In reality, however, the logistics of supplying such a juggernaut in the Three Kingdoms period would have been impossible in any one place. With the Jingzhou marines included, his army would have numbered 200,000. In comparison, Sun Quan had 30,000 stationed at Chaisang. Liu Bei's forces, boosted by the 10,000 men of Liu Qi, could field 50,000 in all. Despite Cao Cao's numerical superiority, his soldiers were far inferior to that of the enemy on the operational level.
In fact, the northern army was at a distinct disadvantage in the ensuring battle, as Zhou Yu pointed out to Sun Quan: "At present the northern lands are not yet pacified, plus Ma Chao and Han Sui roam west of the pass and will surely be Cao Cao's ruin. … It is also cold, and there can be found no straw for horses. His troops travel from the frontiers of China to the rivers and lakes, and are unfamiliar with the terrain; they will surely suffer infectious disease. These four points will be Cao Cao's downfall. Today is the day when my general will be able to capture him." In short, Cao Cao's army suffered from fatigue, unfamiliarity with terrain, uncertainty in the rear and weakness in naval combat. Furthermore, the northern cavalry could not be put to use in the marshy terrain of the south.