Sun Tzu on spies: the use of spies explained

The Art of War ends not with a battle but with a chapter on information — because Sun Tzu believed the war is won or lost long before the armies meet, in what you know.

What Sun Tzu spies teaching comes down to is a single, radical claim: that the whole art of war rests on knowing your enemy, and that knowledge does not fall from the sky. The thirteenth and final chapter of The Art of War, on the use of spies, is where he makes it explicit — foreknowledge is the difference between victory and ruin, and it can only come from people. It is a startlingly modern idea dressed in ancient language, and it reframes espionage not as a dark corner of war but as its very foundation. Here is what he actually said, the five kinds of spies he described, and why it still matters. The full text is worth reading in any translation of The Art of War.

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Why foreknowledge wins wars

"What enables the wise sovereign and the good general to strike and conquer is foreknowledge."

Sun Tzu opens the chapter by insisting that foreknowledge — knowing the enemy's situation in advance — is what separates the great commander from the doomed one. And crucially, he says it cannot be obtained from spirits, omens, or mere calculation; it must be gotten from people who know the enemy. That was a bold rejection of the superstition of his era, and it puts human intelligence at the center of strategy. Everything he taught elsewhere about attacking weakness and winning without fighting depends on this: you cannot exploit what you do not know, which is why spies underwrite the whole idea of knowing your enemy and yourself.

The five classes of spies

The heart of the chapter is Sun Tzu's taxonomy of five kinds of spies, used together. Local spies are recruited from the enemy's ordinary people. Inward spies are enemy officials turned to your side. Converted spies are the enemy's own spies, discovered and won over to feed information back the other way — Sun Tzu considered these the most valuable of all. Doomed spies are given false information and allowed to be caught, so the enemy acts on a lie. And surviving spies are those who return alive with fresh intelligence. When all five work at once, he wrote, the result is a web of knowledge so subtle the enemy cannot fathom it.

Intelligence is not a luxury

"To remain in ignorance of the enemy's condition simply because one grudges the outlay is the height of inhumanity."

Sun Tzu makes a striking moral argument: given how ruinous war is in lives and money, refusing to spend on good intelligence to save a little is not thrift but folly and even cruelty. A campaign can drag on for years and cost countless lives, and the leader who will not invest in knowing the enemy gambles all of it blindly. Information, in other words, is the cheapest and most decisive investment a strategist can make — a theme that runs straight through his leadership teaching.

What it means for modern strategy

Strip away the ancient setting and this chapter is about market and competitive intelligence. In business, the "spies" are honest research: customer interviews, competitor analysis, market signals, the hard look at what your rivals are really doing and what customers actually want. The company that invests in truly understanding its market moves with confidence, while the one that guesses gambles blind — exactly Sun Tzu's point. Winning through knowledge rather than brute force is the same logic that drives all Art of War strategy for business. The lesson is timeless: gather the truth first, and the fight is half won.

Sun Tzu on spies — frequently asked questions

What did Sun Tzu say about spies?

That foreknowledge wins wars and can only come from people — he called the use of spies essential and described five classes of them.

What are the five classes of spies?

Local, inward, converted, doomed, and surviving spies — used together they form a web of intelligence the enemy cannot fathom.

Why did he value intelligence so highly?

Because war is ruinously costly, so grudging a little spending on intelligence while risking everything is, he said, the height of folly.

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