instruction: Could you tell me how Cao Cao's fleet got destroyed during the battle of Red Cliffs based on this paragraph?
context: In 208, Cao Cao marched south with his army hoping to quickly unify the empire. Liu Biao's son Liu Cong surrendered Jing Province and Cao was able to capture a sizable fleet at Jiangling. Sun Quan, the successor to Sun Ce in the lower Yangtze, continued to resist. His advisor Lu Su secured an alliance with Liu Bei, himself a recent refugee from the north, and Zhou Yu was placed in command of Sun Quan's navy, along with a veteran general who served the Sun family, Cheng Pu. Their combined armies of 50,000 met Cao Cao's fleet and 200,000-strong force at Red Cliffs that winter. Cao Cao had to cross the mighty Yangtze River. So when he along with his fleet sailed down the river, many of his men fell to exhaustion, disease and the northerners were not very comfortable on ships and the marshy terrain. So when the two sides met for war at Wulin, Cao Cao wasn’t able to inflict any defeat on the Allied Force. On the other hand, General Huang Gai of Sun Quan’s army came up with a sneaky plan. He sent a forged letter to Cao Cao, stating that he and others wanted to inflict to his side and they would also bring some of the finest ships of the allies. But little did Cao know that the ships were full of flammable reeds. So when they sailed and approached his fleet, they were set alight and they destroyed Cao Cao’s fleet and camps. Seeing the hopeless situation, Cao Cao ordered his army for a difficult retreat through the Marshlands, during which more of his men fell to disease and exhaustion. The allied victory at Red Cliffs ensured the survival of Liu Bei and Sun Quan, and provided the basis for the states of Shu and Wu.
response: Cao Cao's fleet got set on fire by the General Huang Gai. Huang Gai initially informed Cao Cao that he'd betray the Allied Force formed by Liu Bei and Sun Quan. But Huang Gai was actually trying to take this chance to send ships loaded with flammable reeds to Cao Cao's fleet. Cao Cao took the bait and accepted the offer from Huang Gai. Then, when Huang Gai's ships arrived, the Allied Force set Cao Cao's fleet and camps on fire and essentially destroyed Cao Cao's army.