Shandong) and was besieged by Yuan Shang there. Yuan Tan sought help from Cao Cao, and Cao advanced north to attack Ye, forcing Yuan Shang to lift the siege on Pingyuan. In early 204, Yuan Shang incorrectly believed that Cao Cao had withdrawn, so he attacked his brother again at Pingyuan. Cao Cao attacked Ye once more and Yuan Shang headed back to defend his base but was defeated by Cao Cao. Yuan Shang then fled north to Zhongshan (in present-day Shijiazhuang, Hebei), and Ye fell to Cao Cao. Gao Gan also surrendered Bing Province to Cao Cao. During Cao Cao's siege on Ye, Yuan Tan did not help attack Ye but sought to take Yuan Shang's territories, defeating Yuan Shang in Zhongshan. Yuan Shang fled further north to join Yuan Xi in You Province. Cao Cao now accused Yuan Tan of breaching the trust in the alliance so he turned east to attack him, capturing Yuan Tan's last stronghold at Nanpi (南皮; in present-day Cangzhou, Hebei) and killing Yuan. Meanwhile, in You Province, Yuan Xi's subordinate Jiao Chu (焦觸) revolted and surrendered to Cao Cao, forcing Yuan Xi and Yuan Shang to flee further north to join the Wuhuan tribes under chief Tadun. Around this time, Gao Gan also rebelled against Cao Cao but was defeated by 206 and killed while attempting to flee south to join Liu Biao. In 207, Cao Cao's army headed north to attack the Wuhuan, defeating them at the Battle of White Wolf Mountain. Tadun was killed in battle while Yuan Xi and Yuan Shang sought refuge under Gongsun Kang, a warlord who controlled most of present-day Liaoning. Gongsun Kang feared that the Yuans might turn against him and seize his territory, so he had them executed and sent their heads to Cao Cao. By this time, the Yuan clan had been eliminated and much of northern China had been reunified under Cao Cao's control. Developments in southern China (194–208) During his reunification of northern China, Cao Cao did not conduct any major campaigns south while awaiting an opportunity to act against the remaining three most prominent warlords: Sun Quan, who had succeeded his older brother Sun Ce after the latter died in 200; Liu Biao, governor of Jing Province; and Liu Zhang, governor of Yi Province. During that time, Sun Quan was developing his territories in Jiangdong, and strengthening his military forces. In