In 638 BCE, the small state of Song faced the mighty state of Chu at the Hong River, a battle known as the Battle of Hongshui.
When the Song army arrived first and had already formed their battle lines, the Chu army, arriving later, was still scrambling to cross the river. Seeing this, Sima Ziyu, the Right Minister of Song, recognized a golden opportunity to win and urged Duke Xiang of Song, "Your Majesty, the Chu army outnumbers us—they are strong and we are weak. We should launch a surprise attack while they are still crossing and haven't secured their foothold. That way, we can defeat the Chu forces with our smaller numbers."
But Duke Xiang of Song, a staunch believer in Confucian benevolence and righteousness, said, "No! I have heard that a gentleman of virtue does not harm the wounded, does not capture those with graying hair, and does not take advantage of others' peril to bring about their death. If we attack before the Chu army has fully crossed the river, that would violate benevolence and righteousness!"
As they spoke, the Chu army had already crossed the river one after another, but had not yet formed their battle array. Ziyu saw this and again urged Duke Xiang of Song to attack. Duke Xiang of Song shook his head and said, "No! We cannot attack an enemy that has not yet formed its battle array!"
Only after the Chu army had fully crossed the river and formed their battle lines did Duke Xiang of Song finally order the drums to sound for the attack. As a result, the Song forces suffered a crushing defeat, and the duke himself was struck by an arrow in the thigh amid the chaos, fleeing in disgrace.
Later, the idiom "Not Beating an Unformed Battle Array" came to describe the foolish actions of those who rigidly cling to moral principles.
Source: *Han Feizi*, "Wai Chu Zuo Shuo"
Meaning of the Idiom: Later, the Chinese idiom "不鼓不成列" came to describe how the foolish actions of those who rigidly cling to moral principles.