Chapter XIII

Sun Tzŭ establishes that war is tremendously costly, involving massive daily expenditures (1000 ounces of silver), disruption of home production, and impoverishment of 700,000 families when 100,000 soldiers are levied.

Sun Tzŭ observes that hostile armies may face each other for years, yet the decisive battle determining victory occurs in a single day.

It is inhumane and foolish to grudge the small cost of maintaining spies (100 ounces of silver) when ignorance of enemy conditions causes wars to drag on, wasting vastly greater resources.

A ruler who neglects intelligence gathering is not a true leader, cannot aid his sovereign, and cannot achieve victory.

The wise sovereign and good general succeed because they possess foreknowledge of enemy dispositions, which enables them to strike and conquer beyond what ordinary leaders can achieve.

Sun Tzŭ rejects three potential sources of intelligence: divination, inductive reasoning from analogous cases, and mathematical deduction.

The only reliable source of knowledge about enemy dispositions is other human beings—that is, spies.

Sun Tzŭ identifies five types of spies: local spies, inward spies, converted spies, doomed spies, and surviving spies. Using all five creates an undiscoverable intelligence system.

Local spies are inhabitants of the enemy's district employed to gather information.

Inward spies are enemy officials who have been corrupted or coerced into betraying their own side's secrets.

Converted spies are enemy spies who have been captured and bribed into serving our side, providing both intelligence and enabling us to plant false information.

Doomed spies are our own agents who carry false information to the enemy after our spies have been captured and persuaded to relay deceptive reports.

Surviving spies are conventional intelligence agents who return from enemy territory with news and information about the enemy's camp.

Spies deserve the most intimate relationship with commanders, the most generous rewards, and the strictest secrecy regarding their identities and activities.

One must possess intuitive sagacity to use spies effectively, treat them with benevolence and straightforwardness, and possess subtle ingenuity to verify the truthfulness of their reports.

Repeated emphasis on the need for subtlety and the use of spies for all kinds of business activities.

Both the spy who divulges a secret prematurely and the person to whom the secret is told must be executed to prevent further spread of classified information.

Before attempting to crush an army, assault a city, or assassinate a leader, spies must first identify the names and roles of the general's attendants, aides-de-camp, and sentries.

Enemy spies discovered in our midst should be bribed, housed comfortably, and turned to become converted spies serving our purposes.

Information from converted spies allows us to recruit and employ local spies, plant false information through doomed spies, and use surviving spies effectively.

All five varieties of espionage ultimately depend on knowledge derived from converted spies, making their generous treatment essential to successful intelligence operations.

The rise of the Yin dynasty through I Chih and the Chou dynasty through Lü Ya illustrate how former enemy officials, now serving the other side, enabled dynastic overthrows.

Only enlightened rulers and wise generals recognize that using the army's highest intelligence for espionage purposes achieves great results.

Spies are critically important because an army's ability to move depends on the intelligence they provide.