Master Sun says waging large-scale war with 100,000 men costs the people and the nation 1,000 silver taels a day. It causes chaos everywhere and leaves men miserable. It keeps 700,000 families from their work. In war, armies might battle it out over years with no end in sight. How then can a general begrudge the smaller cost of paying for knowledge of his enemy? Such a general is no friend to his men or ruler, and cannot master victory.
Sun quotes a vast sum of money, which is most likely symbolic, to emphasize the great financial cost of war. But he goes on to show the social and human impact war can have. War is a great evil and must only be fought when truly necessary. For each man that joins the war effort, seven families must pull together to support his family, who have most likely lost their strongest laborer. Given the great misery felt across the nation during war, it is economical to pay spies well to end the conflict sooner and return the men to their families. This should be the general’s top concern—to draw war to a speedy conclusion.
Having information early allows rulers and generals to advance and triumph. It brings them uncommon success. This type of knowledge cannot be gained from omens. It can only be gained from men who know the enemy. There are five types of spies: local, internal, double, dead, and live. All of these are mysterious. Local spies are from the enemy state. Internal spies are officials in the enemy’s government. Double agents are enemy spies turned to the general’s side. Dead spies pass on false information to the enemy. Live spies come back with information.
Both rulers and generals can and should hire spies, because accurate knowledge of the enemy and terrain is the most important asset in warfare. Throughout his treatise, Sun has asserted the overwhelming power that comes from gaining knowledge. Now, he explains how to acquire it. He describes in detail each kind of spy, and the type of knowledge that spy has to offer. Most come from the enemy’s camp, so come with varying levels of danger.
No one should be closer to the general than his spies, nor better paid or treated. Wisdom is necessary to managing spies, as are humanity and justice. Without genius and subtlety, the general cannot know their accuracy or truth. Spies have many uses. If the spy tells someone else his information before the general, both spy and listener must be executed.
Hiring and managing spies is no easy task—they are by nature suspicious characters. Yet, the general must keep them as his closest advisor—if they tell anyone their secrets before the general, then they cannot be trusted. The general must be wise and perceptive to know whether to trust their information, and whether it is accurate. That is, he must be wise to gain knowledge in the first place.
When attacking a city or killing someone, the general must know the names of the enemy’s general, servants, and staff. The spies must know to collect all this information in detail. Enemy spies in the general’s camp must be turned and paid well for it, to become double agents. From the double agent can be won local and internal spies. He can also send misinformation to the enemy, and shows the general how to best use live spies.
Sun, who seems to be talking from experience, advises the general to learn every minute detail about the enemy camp, and to be clear in his orders to his spies on this point. The general must know his enemy as well as he knows himself. To ensure this, his spies must be well paid for the information they provide. Compared to the great cost of total war, it is economical to reward spies well—protecting the state coffers in this way creates victory on every front.
The ruler should know all these kinds of spies. He relies on the double agent, so he must be paid and treated well. In the old stories, the Yin dynasty rose because of Yi Zhi, who had first served the Xia. The rise of the Zhou dynasty was thanks to Lyu Ya, who was won over from the Yin. Only worthy rulers and generals can use spies to good effect. Spies are vital in war, and the army’s every move depends on their information.
Sun suggests a diversity of spies is best, yet to locate many types of spies the ruler requires the double agent. This seems a weak point, as he is the least trustworthy spy. Only the exceptionally wise ruler or general can unmask and turn a double agent. Sun gives examples of famous double agents that brought down dynasties—perhaps a warning as much as encouragement as to the usefulness of spies. He suggests that only the virtuous can use spies effectively, referring to the Way, which confers political success on the morally righteous, a commonly held view at the time. Ending on a weighty statement with real-life applications, Sun’s final point stresses the ultimate importance of knowledge in warfare. Without knowledge, the general cannot move his army, cannot plan an attack, and certainly cannot achieve victory.