Liu Biao
In 198, when Cao Cao wanted to launch another attack on a rival warlord, Zhang Xiu, Xun You advised him against it, saying, "Zhang Xiu and Liu Biao share borders. Zhang Xiu and his wandering 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.