The Battle of Guandu-Ending
Cao Cao was preparing to sleep, when he was told he had a visitor named Xu You. The news sent him into such a state of excitement that he could not wait to put his shoes back on, and ran out barefoot to meet his friend. Sitting in Cao Cao's living room, Xu You asked if Cao Cao had a plan ready to stop Yuan Shao's advance, and how much food remained to sustain his army. Cao Cao gave him a deceptive answer, "Enough to last for another year."
Xu You snickered, "I do not think so." "You cannot be fooled, can you? Yes, I only have enough food for six months." Indignant over all the ambiguity, Xu You retorted: "Do you want to defeat Yuan Shao or what? Then you must be far more straightforward with me than you have been."
"One month," was the final and true answer, "after that, we will starve. So what do you think we can do?"
Xu You told his sly friend that Yuan Shao had his supplies and weaponry stored in Wuchao (today's Yanjin, Henan Province) but the place was badly guarded, and a small cavalry would be enough to raid it and burn all Yuan Shao's supplies so that his former lord could be crushed easily.
Cao Cao did exactly what Xu You told him. It was a thorough job and not a single grain was left for Yuan Shao. The fire at Wuchao sent Yuan Shao's forces into a panic. Some officers betrayed Yuan Shao, while those who did not were defeated in a campaign in which Yuan Shao lost his main force. He died two years afterwards. Another seven years passed before Cao Cao cleaned up northern China of all the pro-Yuan Shao forces, and brought it under his control.
* Cao Cao Enforced the Military Law Strictly
In the Battle of Guandu, Cao Cao issued the order "Soldiers who damage wheat will be punished to death" to protect crops. In his several tens of years of military administration life, he issued many orders to punish those who broke laws, committed breach of duty, or deserted in face of battles, and awarded those who made contributions. Moreover, he dealt out the income and production of his own fief to his generals, officials, veterans and the orphans and widows of the dead generals and soldiers.