In the seventh year of the Jian'an Era, in the seventh month, Cao Cao led his army back to Chang'an. To explain how matters had come to this, one had to begin in the third month.
At the start of the year, Cao Cao learned that Liu Bei had marched into Hanzhong, so he personally raised an army of fifty thousand and headed south to stop Liu Bei from taking Hanzhong.
The army departed in the first month and only entered Hanzhong in early March. By then, Liu Bei's forces had already reached the gates of Nanzheng.
Faced with this, Cao Cao first had the great general Ma Chao defeat Liu Bei in battle, then entered Nanzheng and faced Liu Bei in a standoff.
The two armies encamped on opposite sides of the Han River, their linked camps guarding the vital routes up and down the river. Neither side was willing to yield a single step.
Just then, Wu Yi arrived with troops at Mianyang west of Nanzheng. One army stood east and the other west, together amassing eighty to ninety thousand men to surround Nanzheng.
Seeing this, Cao Cao naturally grew anxious. His provisions were running short, and he needed a swift battle, yet the two armies had already settled into a standoff. If this continued, it would be deeply disadvantageous to him.
Thus, he summoned his subordinates to discuss a way to secure victory as quickly as possible. Guo Jia saw this and said:
"Though the two enemy armies stand in the east and west, they fight separately. If we seek swift victory, we must first defeat the weaker one."
"Liu Bei has few troops, but his generals and officers are elite. Liu Zhang's men have never seen war; they should be defeated first."
"East of Mianyang and