In the sixth month of the fifth year of the Jian'an Era, Cao Cao and Yuan Shao clashed at Guandu, and the two sides fought another great battle.
Since the war had begun the previous year, the houses of Cao and Yuan had been locked in friction for more than a year.
Throughout that time, Cao Cao had won small victories while Yuan Shao suffered small defeats, with Cao Cao fighting as he retreated.
Relying on his numerical advantage, Yuan Shao steadily pushed his battle lines forward.
By the time they reached Guandu, Yuan Shao saw Old Cao digging trenches for defense there and discerned Cao Cao's intentions.
Thus, at the suggestion of his adviser Feng Ji, Yuan Shao set up a chain of camps along the northern side of Cao Cao's main camp.
Yuan Shao could be considered a master of civil engineering in the late Han. As he established the connected camps, he also ordered earthen ramparts built as fortifications.
Outside Cao Camp, he raised mounds of earth and erected high towers, attacking Cao Cao's camp from high ground.
He hoped to force Cao Cao to abandon the advantageous terrain and withdraw from Guandu.
Faced with this, Cao Cao's adviser Guo Jia suggested striking back with catapults.
There were not many methods of attack and defense in the late Han. With Liu Ye gone, Guo Jia had taken over Liu Ye's work.
Following the suggestion, Cao Cao hurriedly built catapults through the night and repelled Yuan Shao's offensive.
Seeing this, Yuan Shao returned to his old trade and began digging tunnels to launch a surprise attack on Cao Camp.
But Cao Cao had long prepared for Yuan Shao's three strokes of the axe and simply had the trenches outside his camp dug deeper.
By the sixth month, Yuan