army rely on Liu Biao for supplies. Liu Biao is unable to provide for them so they will eventually fall out. Why not wait and try to induce Zhang Xiu to surrender to you? When Zhang Xiu ends up in a desperate situation, Liu Biao will definitely support him." Cao Cao ignored Xun You's advice and attacked Zhang Xiu at Rang County (穰縣; present-day Dengzhou, Henan). Just as Xun You predicted, when Zhang Xiu's situation became more desperate, Liu Biao sent reinforcements to help him and put Cao Cao's attacking forces in a disadvantageous position. Cao Cao told Xun You that he regretted not listening to his advice. He then changed tactics, attacked Zhang Xiu again, and defeated him the second time. In the same year, after defeating Zhang Xiu and Liu Biao, Cao Cao wanted to move on to attack another rival warlord, Lü Bu. Many of his subordinates thought that it was too dangerous. However, Xun You had a different opinion. He believed that Zhang Xiu and Liu Biao had yet to recover from their recent defeat and would not make any further moves. He also pointed out that even though Lü Bu was a formidable warrior and had support from the warlord Yuan Shu, relations between them had recently deteriorated, so it was an opportune moment for Cao Cao to attack Lü Bu. Lü Bu defeated Liu Bei and received help from Zang Ba. During the Battle of Xiapi, Cao Cao defeated Lü Bu in the initial stages and forced him to retreat back to Xiapi Commandery (下邳郡; south of present-day Pizhou, Jiangsu). Cao Cao then laid siege to Xiapi and launched several attacks but was unable to breach the city walls. As his troops grew weary, Cao Cao considered withdrawing. However, Xun You and Guo Jia advised him, "Lü Bu is brave but foolhardy. His forces' morale is very low after suffering consecutive defeats. An army's morale depends on its commander's will to fight on. Chen Gong is intelligent but slow. Since Lü Bu's army's morale hasn't recovered yet and Chen Gong hasn't finalised his plans yet, you can eventually defeat Lü Bu if you continue attacking him." Cao Cao then ordered his troops to dig ditches and redirect the waters of the Yi and Si rivers to flood Xiapi. Xiapi fell quickly and Lü Bu was captured alive and executed. Battles against the Yuan family