Then Shên Pʻei, the strategist, thought out another plan. He set men to tunnel under the walls into the midst of Tsʻao’s camp and called this corps “The Sappers.” Tsʻao’s men saw the enemy digging out pits behind the mounds and told the chief, who at once sought a counter plan from Liu Yeh. “As Yüan Shao can no longer attack openly he is attacking secretly and is tunnelling a road under ground into the midst of our camp,” said he. “But how to meet it?” “We can surround the camp with a deep moat which renders their tunnel useless.” So a deep moat was dug as quickly as possible and when the enemy sappers arrived thereat, lo! their labour had been in vain and the sap was useless. It is recorded that Tsʻao held Kuantu throughout the eighth and ninth months when, the men being worn out and provisions failing, he began to think of giving up and returning to the capital. As he could not make up his mind he referred his difficulties by letter to Hsün Yü, whom he had left to guard Hsüchʻang. The reply he got was to this effect:—“I have received your command to decide whether to continue the campaign or retire. It appears to me that Yüan Shao assembled such large forces at Kuantu with the expectation of winning a decision. You, Sir, are very weak while he is very strong and if you cannot get the better of him he will be able to work his will on you for this is a crisis of empire. Your opponents are indeed numerous, but their leader knows not how to use them. With your military genius and discernment where are you not sure to succeed? Now though your numbers are small it is not as when Chʻu and Han were between Jungyang and Chʻêngkao. You are securely intrenched with your hands on his throat, and even if you cannot advance, that state of things cannot endure for ever but must change. This is the time to play some unexpected move and you must not miss it. The device I leave to your illustrious ingenuity.” This letter greatly pleased Tsʻao and he urged upon his men to use every effort to maintain the position. Yüan having retired some thirty li, Tsʻao sent out scouts to ascertain his new dispositions. One of Hsü Huang’s officers captured an enemy spy and sent him to his chief, who interrogated him and found out that a convoy of supplies was expected and that this spy and others had been sent to find out what were the risks of the route. Hsü Huang went at once to tell Tsʻao. When the adviser Hsün Yu heard that the commander of the convoy was Han Mêng he said, “That fellow is a valiant fool. A few companies of light horse sent to intercept him can capture the whole train and cause much trouble in the enemy’s camp.” “Whom should I send?” asked Tsʻao. “You might send Hsü Huang; he is capable of such a task.” So Hsü was deputed and he took with him Shih Huan, who had captured the spy, and his company. And this party was supported by Chang Liao and Hsü Chʻu.