Xun Yu initially served under Yuan Shao, but he left to join Cao Cao in Yanzhou. Cao Cao gladly accepted him and declared him his 'teacher'. As Cao Cao's chief of staff, Cao Cao praised him as "my Zhang Liang"; a famous tactician of the old days. At that time, Dong Zhuo ruled like a despot. Xun Yu remarked, "Dong Zhuo has reached the limits of atrocity, I am certain that his barbarity will lead to his death. When Cao Cao left Yanzhou in an expedition to overthrow Tao Qian, his district was attacked by Zhang Miao and Lu Bu. Xun Yu and Cheng Yu were left behind in those unguarded moments, but they managed to hold three castles.
     Cao Cao was worried when Lu Bu escaped to Liu Bei in Xuzhou. He appealed to his staff of advisors, and Xun Yu readily contrived the Rival Tigers Ruse. "Send Liu Bei an imperial decree to give him the right to rule in Xuzhou, and enclose a secret letter. Herein you must order him to oust Lu Bu. If he tries and fails, Lu Bu will undoubtedly kill Liu Bei. No matter how the scenario evolves, your foes will weaken each other, and you will gain an edge." However, Liu Bei saw through the treachery in Cao Cao's letter and did not approach Lu Bu.
     Xun Yu had yet another scheme for his lord to put to use, the Tiger-Wolf Trick. "I have a plan in which the tiger is forced to devour the wolf. Order Liu Bei to attack Yuan Shu. While Liu Bei is away, Lu Bu will betray him and take over Xuzhou." Cao Cao certified his letter with the emperor's seal. Unable to refuse an order from the throne, Liu Bei set off to challenge Yuan Shu. Lu Bu took advantage of his absence and usurped Xuzhou. Xun Yu's plan successfully marred the relationship between Liu Bei and Lu Bu.
     Later Xun Yu convinced Cao Cao to attack Yuan Shao. He said, "Yuan Shao's great army is in shambles. His generals are completely at odds. For example, Tian Feng is brave but distrust, Xu You is covetous and evil, Shen Pei is strict yet simple-minded, Feng Ji is stubborn and egocentric. And, Xu You is going to betray them all. Yan Liang and Wen Chou are merely common soldiers in general's suits, they will crumble at the suggestion of war." He read the enemy for what they really were.
     Finally, in the Battle of Guandu, Cao Cao's men were in danger of running out of provisions. However, Xun Yu warned against retreat. "Withdrawal is the same as admitting defeat. Channel your strength into a new plan." His words inspired Cao Cao to renew his attack, and he eventually devastated Yuan Shao. Sensitive to changes in the heavens, and speculative of events yet to come, Xun Yu was a brilliant tactician and a man of infallible character.