Around this time, Cao Cao received intelligence that Tian Feng, an adviser to his rival Yuan Shao, had suggested that Yuan Shao should take advantage of Cao Cao's absence from Xu to launch an attack on the imperial capital, seize Emperor Xian, and bring him to his base in Ye (鄴; in present-day Handan, Hebei). Upon hearing this, he immediately lifted the siege on Rang County and prepared to return to Xu. However, he could not retreat as Zhang Xiu came to intercept him, so he ordered his troops to retreat carefully while ensuring that their camps were always linked so that they could back each other up in the event of an enemy attack. Cao Cao wrote a letter to his adviser Xun Yu, who was stationed in Xu: "Even though the enemy has travelled several "li" in a day to catch up with me, I have a plan to deal with them. When I reach Anzhong, I will definitely defeat Zhang Xiu." When he reached Anzhong County (安眾縣; in present-day Dengzhou, Henan), Zhang Xiu and Liu Biao's forces occupied the strategic locations in front and behind, trapping him and his forces in between. Cao Cao then ordered his troops to secretly dig tunnels and transport their supplies and heavy equipment back to Xu under the cover of night, while he and his remaining troops hid themselves and waited in ambush.