During the early Warring States period, King Hui of the state of Chu wished to restore Chu's former dominance. He expanded his army with plans to attack the state of Song. King Hui greatly valued a master craftsman of the time, a man from the state of Lu named Gongshu Ban—later known as Lu Ban. Gongshu Ban's skill with an axe was unmatched; anyone who tried to compete with him in axe work was clearly overestimating their own ability. This is the origin of the idiom "to show off one's axe before Lu Ban" (班門弄斧), meaning to display one's skill where an expert is present.
Gongshu Ban was summoned by King Hui and appointed as a high official in Chu. He designed a siege device for the king, even taller than the existing siege towers. It was so high it seemed to touch the clouds, hence its name: the "Cloud Ladder."
While King Hui ordered Gongshu Ban to hasten the construction of the Cloud Ladder, he also prepared to launch an attack on Song. News of Chu's Cloud Ladder project spread quickly, causing concern among the rulers of other states. The people of Song, in particular, felt as if disaster was upon them.
The planned invasion of Song also drew opposition from some individuals, most strongly from Mozi. Mozi, whose given name was Di, was the founder of the Mohist school of thought. He opposed extravagance and wastefulness, advocating instead for frugality. He required his disciples to wear simple clothes and straw sandals, to participate in manual labor, and to regard enduring hardship as a noble pursuit. To avoid hardship was, in his view, a violation of his principles. Mozi also opposed the chaotic wars waged by rulers to seize territory at the expense of the people's suffering. When he learned that Chu intended to use the Cloud Ladder to invade Song, he immediately set out on foot for Chu. His feet blistered and bled from the journey, so he tore strips from his clothes to bandage them and kept walking. After ten days and ten nights of relentless travel, he arrived at Ying, the capital of Chu. His first stop was to see Gongshu Ban, urging him not to assist King Hui in attacking Song.
Gongshu Ban replied, "I'm afraid I cannot. I have already promised the king."
Mozi then requested that Gongshu Ban take him to meet King Hui, which Gongshu Ban agreed to do. Before King Hui, Mozi spoke earnestly: "Chu is vast, with territory spanning five thousand li, rich in land and resources. Song, by contrast, covers only five hundred li, with poor soil and meager produce. Why would Your Majesty, already possessing fine chariots and horses, desire to steal a broken-down cart? Why would you discard your own embroidered silk robe to steal a tattered, old short jacket?"
Though King Hui found Mozi's words reasonable, he still refused to abandon his plan to attack Song. Gongshu Ban also remained confident that the Cloud Ladder would ensure victory.
Mozi responded bluntly, "You may attack, but I can defend—and you will gain no advantage."
He then removed his belt and laid it on the ground to represent the city walls. Taking several small wooden boards as siege equipment, he challenged Gongshu Ban to a simulated battle to test their skills.
Every time Gongshu Ban used a method to attack, Mozi had a countermeasure to defend. When one used a Cloud Ladder to scale the walls, the other used fire arrows to burn it. When one deployed a battering ram against the gate, the other dropped logs and boulders to crush it. When one tried tunneling, the other countered with smoke.
Gongshu Ban exhausted all nine of his siege tactics, yet Mozi still had numerous defensive strategies left unused.
Gongshu Ban was stunned, though still不服 in spirit. "I have a way to defeat you," he said, "but I won't say it now."
Mozi smiled faintly. "I know what you're thinking, but I won't say it either."
King Hui, hearing this cryptic exchange, was utterly confused. "What exactly are you two talking about?"
Mozi explained, "Gongshu Ban's intention is clear: he wants to kill me, believing that without me, Song would have no one to help defend its city. But he has miscalculated. Before I came to Chu, I had already sent three hundred disciples, including Qin Huili, to guard the walls of Song. Each one has mastered my defensive techniques. Even if you kill me, Chu will still gain no advantage."
After hearing Mozi's words and witnessing his defensive prowess firsthand, King Hui realized there was no hope of defeating Song. He finally said, "Master, your words are wise. I hereby renounce my plan to attack Song." Thus, Mozi single-handedly prevented a war.