English: Cao Cao's conquest of northern China 200–207. This is an episode during the end of the Han dynasty, in the prelude of the Three Kingdoms period. Cao Cao and Yuan Shao, who had been childhood friends, had become the two most powerful warlords in China by the start of the year 200 amid growing rivalry. Despite his numerical superiority, Yuan Shao was defeated during the Guandu campaign (February 200 – June 201) and died in June 202, after which his three sons Yuan Tan, Yuan Shang and Yuan Xi entered into a war of succession. Cao Cao exploited the disunity in the Yuan clan by allying with Yuan Tan in 203, conquered the Yuan 'capital city' of Ye (controlled by Yuan Shang) in 204, killed Yuan Tan in the Battle of Nanpi in 205, defeated and killed Gao Gan (who had served the Yuans until 204, and served Cao Cao until rebelling in 205) in 206, and eventually defeated Yuan Xi, Yuan Shang and the Wuhuan tribes in the Battle of White Wolf Mountain in 207.