Original Text
Sun Tzu said: When raising an army of one hundred thousand and marching a thousand miles, the people's expenses and the state's outlays amount to at least a thousand pieces of gold each day. Both at court and in the field, there is turmoil; laborers and soldiers are exhausted from constant movement, and as many as seven hundred thousand households are unable to tend to their fields. Such a stalemate may last for years, all for the sake of a single day's victory. Yet if, in the end, one begrudges ranks and rewards, refusing to employ spies to gain knowledge of the enemy, and thus fails and suffers defeat, this is the height of inhumanity. Such a person is unfit to be a commander of an army, unworthy to be an advisor to a ruler, and incapable of achieving ultimate victory. The enlightened ruler and the wise general who conquer every time they move and achieve feats beyond the ordinary do so because they gain prior knowledge of the enemy. This knowledge cannot be obtained from spirits or gods, nor deduced by analogy, nor verified by the movements of the sun, moon, and stars. It must be acquired from those who truly know the enemy's situation.
There are five types of spies: local spies, internal spies, double spies, doomed spies, and surviving spies. When all five are used together, the enemy cannot fathom the pattern of your espionage—this is the mysterious and subtle art of employing spies, and the sovereign's key to victory. Local spies are ordinary people from the enemy's own populace. Internal spies are enemy officials recruited to work for you. Double spies are enemy spies turned to your service. Doomed spies are those who spread false information through your own agents to deceive the enemy; once the truth is exposed, they are inevitably killed, hence their name. Surviving spies are those who return alive to report the enemy's situation.
Therefore, for the sovereign and commanding general who lead the entire army into battle, no one in the ranks is more worthy of trust than the spy, no reward in the army is more generous than that given to the spy, and no military secret is more confidential than the use of spies. Only the wise and enlightened can employ spies; only the benevolent and just can command them; only the subtle and discerning can analyze the truth of the intelligence they provide. Subtle, subtle! There is no place or time where spies cannot be used in the course of operations. If the spy's work is compromised before it begins, both the spy and those who know the secret must be executed.
When planning to attack an enemy force, seize a city, or assassinate an enemy leader, you must first know the enemy's commanders, their trusted advisors, the officers who relay reports, the gatekeepers, and their retainers and staff. To obtain this intelligence, you must command your own spies to investigate thoroughly.
For enemy spies sent to gather intelligence on us, they must be uncovered. Then, treat them generously with wealth and rewards, bribe them, guide them with persuasion, and release them back. In this way, they can be turned into double agents, serving our purposes. By learning the enemy's situation through double agents, we can cultivate and employ local agents and internal agents. Likewise, based on intelligence from double agents, we can use dead agents to spread false information to the enemy. And because we understand the enemy's situation through double agents, avoiding danger, living agents can return with their reports on schedule. A commander must know and master all five types of agents, but the key to understanding lies in using double agents. Therefore, double agents must be treated with the utmost generosity.
In ancient times, the rise of the Shang dynasty was due to Tang employing Yi Yin, who had served as a minister in the Xia court and understood its affairs. The rise of the Zhou dynasty was due to King Wu employing Jiang Ziya, who understood the conditions of the Shang. Therefore, a wise ruler and a capable general, if they can employ highly intelligent people as spies, will surely achieve great success. This is the key to military operations and the foundation of an army's actions.
Commentary
Sun Tzu's five methods of using spies can be described as highly comprehensive. There appear to be five ways to employ spies, but in practice, their applications are endlessly varied, depending on one's skill in using them. Since ancient times, the use of spies has not been limited to warfare; it has been widely applied in diplomacy, economics, and many other fields, giving rise to numerous military texts and strategies on espionage. Even the Thirty-Six Stratagems includes a specific stratagem for using double agents.
Example Interpretation
Yue Fei used a counter-intelligence scheme to depose Liu Yu.
One of the spy's tasks is to sow distrust within the enemy camp; counter-espionage involves exploiting the enemy's plot to divide us and turning it against them. Using counter-espionage carries great risks and crises, sometimes requiring long-term preparation and planning, while at other times it can be quite accidental—it may suddenly appear or vanish in an instant. This demands seizing the right moment to execute the counter-espionage scheme.
Yue Fei was not only skilled in warfare but also rich in stratagems, often catching his enemies off guard. He frequently employed the ruse of sowing discord among the enemy, and each time he reaped rewards.
In the second year of the Jian'an era of the Southern Song, the Jin army invaded the south and besieged Jinan. Prefect Liu Yu killed the anti-Jin general Guan Sheng and surrendered to the Jin. Two years later, he was appointed by the Jin ruler as the puppet emperor of the Great Qi state.
At that time, Yue Fei stationed his troops in Jiangzhou (present-day Jiujiang, Jiangxi). Upon learning that Zongbi, the fourth son of Emperor Taizu of Jin (also known as Jin Wuzhu), deeply despised Liu Yu, he believed the conflict between the two could be exploited to eliminate Liu Yu.
One day, Yue Fei's army captured a spy under Jin Wuzhu's command. Yue Fei pretended to recognize him and scolded loudly, "Aren't you Zhang Bin from my army? I sent you to deliver a letter to Qi earlier. Liu Yu promised this winter to lure the Fourth Prince to Qinghe under the pretext of a joint attack on the Yangtze River. Why did you never return?" Hearing this, the spy thought Yue Fei had mistaken him for someone else. To save his life, the spy went along with the ruse, claiming to be Zhang Bin and begging Yue Fei for mercy.
Yue Fei saw that the spy had taken the bait, so he quickly wrote a letter to Liu Yu, detailing a plan to secretly assassinate Jin Wuzhu. Then Yue Fei said to the spy, "I will spare you this time and give you a chance to earn merit by going to the Qi state." He then cut open the spy's thigh, placed the sealed letter inside, and warned him not to reveal it. After the spy returned, he handed the letter to Jin Wuzhu, who was greatly alarmed and immediately sent it to the Jin ruler, Wanyan Sheng. At that moment, someone reported that Liu Yu had been secretly communicating with the Southern Song chancellor, possibly plotting against the Jin. Wanyan Sheng believed it without doubt and promptly arrested Liu Yu, imprisoning him in Jinming Pool. The puppet Qi regime thus came to an end, and Yue Fei's counter-espionage scheme achieved great success.