During the Spring and Autumn period, after relentless wars of conquest, six great powers emerged: Chu, Qi, Qin, Jin, Wu, and Yue.
King Helü of Wu, ambitious and ruthless, seized the throne by sending the assassin Zhuan Zhu to murder King Liao of Wu. Once in power, he expanded his influence, appointing the exiled Chu minister Wu Zixu to reform the state. As Wu grew strong, Helü appointed the military strategist Sun Wu as his general to launch a massive invasion of Chu. They won five consecutive battles, captured Chu's capital Ying, and plundered its people, artifacts, and treasures.
A few years later, the state of Chu had regained its strength and sought revenge, dispatching a hundred thousand troops to attack the state of Wu.
At this point, the King of Wu, unsure of Chu's military strength, wanted to send someone to scout their true situation, but it was a dangerous mission. Gathering his ministers, Wu Zixu proposed sending two men disguised as commoners to present gifts to Chu's troops. Though the plan was feasible, it still carried mortal risk. Seeing no one willing to volunteer, the King of Wu promised a heavy reward for anyone who succeeded.
When a heavy reward is offered, brave men will step forward. Two men, Zu Wei and Jue Rong, volunteered for the mission. After careful planning, they brought dozens of loads of wine and food to the Chu army camp, requesting an audience with the Chu general, claiming to be Wu citizens coming to reward the troops.
The Chu general thought, how could such a good thing exist in the world
"Tie up these two men from Wu! We'll use their blood for the drum consecration ritual!" the Chu general roared. Zu Wei and Jue Rong tensed but forced calm. "General, please don't be hasty," Zu Wei replied. "Let us finish speaking before you kill us." Jue Rong glanced at Zu Wei and whispered, "Could our divination be wrong? How did we run into such ill fortune here?" The Chu general, intrigued, asked, "Did you consult the oracle before coming?" "We did," they answered. "Was it auspicious?" "It was." The general laughed heartily. "Now that I'm about to kill you both for the ritual, how can that be auspicious?" Zu Wei replied, "Precisely because you will kill us for the drum consecration—that is exactly why it is auspicious!"
The Chu general was utterly bewildered.
Zu Wei continued, "The King of Wu sent us specifically to test whether you, General Chu, would show anger. If you grew angry, Wu would dig deep moats, build high walls, and prepare for battle. If you did not, we would relax our guard. Now that you have killed two of us, Wu will surely fortify its defenses. Our troops are elite, our granaries full, and our strength greater than ever. The choice is yours."
The Chu general fell silent.
"Besides, I divined for the nation, not for myself alone. If you kill me, though it may be unlucky for me, if my death can preserve the kingdom, isn't that a great fortune for the state of Wu?"
General Chu pondered. Zu Wei pressed on, "Besides, if the dead are senseless, my blood on your drum does you no good. If they have awareness, I will surely make your drum silent."
The Chu general, upon hearing this, found it reasonable and thought, "Even if I were to kill him and use his blood to anoint the war drums, if the drums still wouldn't sound, how could the army have any morale to attack?"
Thus, the Chu general spared the two men.
"The idiom 'Deep Trenches and High Walls' literally means digging deep moats and building high ramparts, later used to describe constructing solid defensive fortifications."
Source: *Han Feizi*, "Shuo Lin Xia"
Meaning of the Idiom: Later, the Chinese idiom "深沟高垒" came to describe constructing solid defensive fortifications.