Yuan Shao adopted this suggestion。 From each of the camps； they sought out the strongest veterans who dug with iron spades and carried earth to raise mounds near Cao Cao's camp。
Cao Cao's soldiers saw what their enemies were doing and were anxious to make a sortie and drive them off。 But the archers and crossbowmen came out manding the narrow throat through which it was necessary to attack and stayed them。 At the end of ten days； they had build up more than half a hundred mounds； and on the summit of each was a lofty tower； whence the archers could mand their opponents' camp。 Cao Cao's soldiers were greatly frightened and held up their bucklers to keep off the various missiles。 From the mounds the arrows flew down like a fierce rain after each roll of drums。 The soldiers of Yuan Shao's army laughed and jeered when they saw their enemies crouching under their shields and crawling on the ground to avoid their missiles。
Cao Cao saw that his troops were getting out of hand under this attack； so he called a council。
Liu Ye spoke up； saying； 〃Let us make catapults and so destroy them。〃
Cao Cao at once had models brought and set cunning workers to make these stone…throwing machines。 They soon constructed some hundreds and placed them along the walls of the camp inside； just opposite the high ladders on the enemy's mounds。
Then Cao Cao's troops watched for Yuan Shao's archers to ascend the towers。 As soon as the archers began to shoot； all the catapults began to heave stone balls into the skies and they wrought great havoc。 There was no shelter from the falling stones； and enormous numbers of the archers were killed。 Yuan Shao's troops called these machines 〃Rumblers；〃 and after their appearance the archers dared not ascend the mounds to shoot。
Then Shen Pei； the strategist； thought out another plan。 He set troops to tunnel under the walls into the midst of Cao Cao's camp and called this corps 〃The Sappers〃。 Cao Cao's soldiers saw the enemy digging out pits behind the mounds and told the chief； who at once sought a counter plan from Liu Ye。
〃As Yuan Shao can no longer attack openly； he is attacking secretly and is tunneling a road under ground into the midst of our camp；〃 said Liu Ye。
〃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 labor had been in vain and the sap was useless。
Cao Cao held Guandu throughout the eighth and ninth months when； his army 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 Xun Yu； whom he had left to guard Xuchang。 The reply he got was to this effect：
〃I have received your mand to decide whether to continue the campaign or retire。 It appears to me that Yuan Shao assembled such large forces at Guandu with the expectation of winning a decision。 You； Sir； are very weak while he is very strong。 If you cannot get the better of him； he will be able to work his will on you； and this will be a crisis of the 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； your situation is still brighter than Liu Bang's when he faced against Xiang Yu in Jungyang and Chenggao。 You are securely entrenched with your hands on Yuan Shao's throat； and even if you cannot advance； that state of things cannot endure forever 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 Cao Cao； and he urged upon his troops to use every effort to maintain the position。
Yuan Shao then retired some ten miles； and Cao Cao sent out scouts to ascertain his new dispositions。 One of Xu Huang's officers； Shi Huan； captured an enemy spy and sent him to his chief。 Xu Huang 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 the risks of the route were。 Xu Huang went at once to tell Cao Cao。
When Xun You heard that the mander of the convoy was Han Meng； he said； 〃That fellow is a valiant fool。 A few thousand 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 Cao Cao。
〃You might send Xu Huang。 He is capable of such a task。〃
So Xu Huang was deputed； and he took with him Shi Huan； who had captured the spy； and his pany。 And this party was supported by Zhang Liao and Xu Chu。
It was night when the missariat train of many thousands of wagons drew near Yuan Shao's camp。 As they passed through a defile； Xu Huang and Shi Huan came out and stopped the train。 Han Meng galloped up to give battle but was soon overe。 The guard was scattered； and soon the whole train was in flames。 The escort and their leader fled away。
The glow of the flames seen from Yuan Shao's camp caused great consternation； which became fear when the escaped soldiers rode in and told their tale。
Yuan Shao sent out Zhang He and Gao Lan to try to intercept the raiders； and they came upon Xu Huang and his pany。 Just as Zhang He and Gao Lan were attacking； reinforcements from Zhang Liao and Xu Chu came up； and the Yuan Shao's troops were between two fires。 They were cut to pieces and the successful generals of Cao Cao rode back to Guandu； where they were richly rewarded。
As an additional safeguard； Cao Cao made a supporting outpost in front of the main camp to be the apex of a triangle of defense。
When Han Meng returned with his woeful tidings； Yuan Shao was angry and threatened to put him to death。 His colleagues begged him off。
Then said Shen Pei； 〃Food is very important for an army in the field and must be defended with the greatest diligence。 Wuchao is our main depot and must be carefully guarded。〃
〃My plans being plete；〃 said Yuan Shao。 〃You may as well return to Capital Yejun of Jizhou and undertake the control of the supplies。 Let there be no shortage。〃
So Shen Pei left the army。 Then a force of twenty thousand troops was told off to defend the depot in Wuchao。 The leaders of this body were Chunyu Qiong； Gui Yuanjin； Han Juzi； Lu Weihuang； and Zhao Rui。
Of these generals； Chunyu Qiong was a hard man and a heavy drinker； who in his cups was a terror to the soldiers。 Under the idle life of guarding the supply depot； the leaders gave themselves up to indulgence and drank heavily。
In Cao Cao's army food was also getting scarce； and a message was sent to Capital Xuchang to send grain quickly。 The messenger with the letter； however； had not gone far when he fell into the hands of Yuan Shao's guards； who took him to the adviser Xu You。
Seeing from the letter that Cao Cao was short of supplies； Xu You went to Yuan Shao and told him； saying； 〃Cao Cao and we have been at grips here for a long time； and Capital Xuchang must be undefended。 A small army sent quickly could take it； and at the same moment an attack here would deliver Cao Cao into our hands。 Now is the moment to strike； for his supplies are short。〃
Yuan Shao replied； 〃Cao Cao is full of ruses； and this letter is artfully designed to bring about a battle to suit himself。〃
〃If you do not take this chance； he will do you some injury by and by。〃
Just at this juncture in came a dispatch from Yejun in which； after some details regarding the forwarding of grain； Shen Pei said he had discovered that Xu You had been in the habit of receiving bribes while in Jizhou and had winked at his relatives collecting excess taxes。 One of his son and nephew were then in prison。
At this Yuan Shao turned on Xu You angrily and said； 〃How can you have the face to stand before me and propose plans； you extortionate fellow？ You and Cao Cao have old liking for each other； and he has bribed you to do his dirty work for him and help his base schemes。 Now you want to betray my army。 I ought to take off your head； but temporarily I will let your neck carry it away。 Get out and never let me see you again。〃
The discredited adviser sighed and went out； saying； 〃