Treatise on Armament Technology, Volume 39: Strategic Analysis (Tang III)

The Imperial Army collapsed and fled to the south, while the rebel army collapsed and fled to the north The "rebels" refers to the forces of the Great Yan state, led by Shi Siming, during the later stages of the An Lushan Rebellion. Amidst this chaos, the generals Li Guangbi and Wang Sili managed to reorganize their troops and began their return march.

Shi Siming divided his army into four separate routes. They crossed the Yellow River and converged at Bianzhou modern-day Kaifeng, Henan. Li Guangbi was in the middle of inspecting various military camps when he heard of the rebel advance. He entered Bianzhou and spoke to the Military Governor, Xu Shuji: “If you, Excellency, can hold Bianzhou for fifteen days, I will then lead my troops to come to your rescue.” Xu Shuji gave his word. However, when Shi Siming reached Bianzhou, Xu Shuji fought but could not secure a victory; he subsequently surrendered to the rebels.

Following this victory, Shi Siming took advantage of the momentum and attacked Zhengzhou to the west. Meanwhile, Li Guangbi reorganized his followers and marched at a deliberate pace until he reached Luoyang.

The loss of Bianzhou was a significant blow, as Xu Shuji was a veteran commander who chose survival over a hopeless defense.

The Regent of Luoyang, Wei Zhi, requested that they leave a garrison at Shanzhou and retreat further to defend Tongguan Pass Tongguan was the primary mountain pass guarding the way to the Tang capital, Chang'an. Li Guangbi disagreed, stating: “When two enemies are evenly matched, one must value the advance and dread the retreat. To abandon five hundred li of territory approximately 200–250 kilometers now without cause would only allow the rebels’ power to expand further. It would be better to move the army to Heyang. To the north, this position connects with the regions of Zezhou and Luzhou. If conditions are favorable, we can advance to take territory; if they are unfavorable, we can retreat and defend. The front and the interior will support each other in a coordinated response. This will ensure the rebels do not dare to invade further westward. This is like the ape... original: "此猿" (ci yuan) — likely the beginning of a strategic metaphor comparing military positioning to the agility or the "long arms" of a gibbon