Guo Tu
As soon as Cao Cao started to retreat in mid-203, the brothers' feud took a turn to the worse. Yuan Tan requested more troops and equipment so he could lead an army to pursue Cao Cao's men, but Yuan Shang refused, not wanting his brother to gain control of a bigger army. This act of distrust proved to be the last straw for Yuan Tan, as he rebelled from his younger brother and attacked the city of Ye in anger. Yuan Shang successfully defended Ye again and chased Yuan Tan 300 kilometers away to Nanpi, the seat of Bohai Commandery. Bohai was on the edge of the border with Qing Province, where Yuan Tan still held the title Inspector of that province; but while he could expect some aid from the base of his power, some of his own men rebelled against him, and the position was very insecure. So when Yuan Shang came to attack Nanpi, Yuan Tan fled south to Pingyuan, where he was again besieged. Here he was close to Cao Cao's Yan province, and his advisor Guo Tu suggested seeking help from Cao Cao. Guo Tu reasoned that Cao Cao's army would attack Ye, and while Yuan Shang was to return to save his capital, Yuan Tan could expect to take the lands to the north of Ye; and if Cao Cao was to be successful, Yuan Shang's power would be removed and the outstretched Cao Cao would retreat soon after, leaving Yuan Tan to gain control of the north and capable enough to make a stand against Cao Cao. Yuan Tan would not agree at first, but later sent Xin Pi as an ambassador to seek an alliance with Cao Cao.