Cheng, Liu Bei and a few other officials were conspiring against him. In early 200, Liu Bei used an opportunity to break away from Cao Cao and seize control of Xu Province, after killing Cao Cao's appointed governor of the province, Che Zhou (車冑). On the other hand, Dong Cheng and the others were making plans to assassinate Cao Cao. However, the plot was exposed and all the conspirators in the capital were massacred along with their families. Cao Cao then took a risk by attacking Liu Bei in Xu Province, leaving his flank open to attacks from Yuan Shao. However, Cao Cao made the right choice because Yuan Shao did not heed Tian Feng's urging to seize the chance to attack him. Liu Bei was defeated and he fled north to join Yuan Shao. Liu Bei's general Guan Yu surrendered to Cao Cao and temporarily served under Cao. Only after Liu Bei's defeat did Yuan Shao start implementing his plan to attack Cao Cao, but this time Tian Feng opposed his decision, saying that their opportunity had passed. Yuan Shao became annoyed with Tian Feng and had Tian imprisoned, after which he led his army south to attack Cao Cao. At the Battle of Boma, Yuan Shao's general Yan Liang was slain by Guan Yu while another of Yuan's generals, Wen Chou, was killed in action against Cao Cao's forces. The morale of Yuan Shao's army was greatly affected by the loss of the two generals. By late 200, the armies of Yuan Shao and Cao Cao finally clashed at Guandu (官渡; in present-day Zhengzhou, Henan), south of the Yellow River. Yuan Shao had two advantages over Cao Cao - numerical superiority and a greater amount of supplies, but Cao Cao's troops were better trained than his. After some minor skirmishes, both sides became locked in a stalemate, until Cao Cao personally led a small detachment on a surprise attack on Yuan Shao's supply depot at Wuchao, which was defended by Chunyu Qiong. Instead of sending reinforcements to Wuchao, Yuan Shao sent Zhang He and Gao Lan (高覽) to attack Cao Cao's camp but was unsuccessful. The fall of Wuchao dealt a major blow to the morale of Yuan Shao's army, which was subsequently routed by Cao Cao's forces. Yuan Shao fled north of the Yellow River while most of his troops were either killed or surrendered to Cao