Battle of Guandu
Yuan Shao challenged Cao Cao for control over the emperor. Yuan's power was greatly diminished after Cao defeated him at the Battle of Guandu in 200 AD. After Yuan died, Cao killed Yuan Shao's son Yuan Tan (173–205 AD), who had fought with his brothers over the family inheritance. His brothers Yuan Shang and Yuan Xi were killed in 207 AD by Gongsun Kang (d. 221 AD), who sent their heads to Cao Cao.