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.