Yuan Tan
Cao Cao was engaged in battle with Liu Biao on their common border when Yuan Tan's ambassador Xin Pi came to him. It turned out that Xin Pi was disillusioned about his lord, and suggested to Cao Cao that this would be the opportunity to destroy both Yuan Shang and Yuan Tan, before the two brothers make up and unite their forces. Xun Yu had also made an argument along these lines previously. Cao Cao accepted the advice, and ostensibly allied with Yuan Tan. In 204, Cao Cao launched an attack from Liyang and routed Yuan Shang's army from Ye to relieve Yuan Tan, causing Yuan Shang to seek refuge with Yuan Xi. In the year after that, Cao Cao accused Yuan Tan of ill intent and cancelled the alliance, and followed that by laying siege to Nanpi. Yuan Tan was killed in that battle. The Yuan clan's hold on China's north was thus broken, though they would hold on until their final destruction in 207.