Comprehensive History Tongzhi: a massive 12th-century historical work by Zheng Qiao, Volume 28

"Jingzhou is prosperous and peaceful, and that state currently has no cause for internal strife. Meanwhile, the two Yuan brothers Yuan Tan and Yuan Shang, sons of the late Yuan Shao do not concern themselves with long-term strategy, but instead plot against one another. Their situation is so precarious they cannot plan from morning until evening; the people’s lives have no stability. If you do not pacify them now, but wish to wait for another year—in another year, there might be a good harvest, or they might realize their ruin is coming and reform their virtues. Should that happen, you will have lost the essential opportunity for military action.

Now, by taking advantage of their request for rescue to pacify them, there is no greater benefit. Furthermore, among the enemies of the four directions, none are greater than those in Hebei. Once Hebei is pacified, then your six armies a traditional term for the Emperor's or a great leader's full military force will be powerful, and the world will tremble."

Cao Cao said, "Excellent." He then granted Yuan Tan's request for peace. In the tenth month, he arrived at Liyang. When Yuan Shang heard that Cao Cao had crossed the Yellow River, he abandoned his siege of Pingyuan and returned to Ye. Cao Cao then withdrew his troops.

In the first month of the following year, Cao Cao crossed the river. He dammed the waters to divert them into the Baigou literally "White Canal" to open a grain transport route. In the second month, Yuan Shang again attacked Yuan Tan at Pingyuan, leaving Shen Pei to guard Ye. Cao Cao constructed earthen mounds and subterranean tunnels referring to siege works and sapping to attack...