The final chapter argues that information is the most decisive weapon. The cost of an army in the field is enormous; the cost of intelligence to direct that army is trivial by comparison. Yet most commanders spend lavishly on the first and refuse to spend on the second, and they lose.
Sun Tzu names five kinds of spies: local (recruited from the enemy's region), inward (from the enemy's officials), converted (the enemy's spies turned), doomed (sent with false information to be captured), and surviving (those who return). The masterful commander uses all five at once, and only the most enlightened ruler can manage them.
The deeper claim: nothing is harder than the use of intelligence, and nothing is more rewarded. An enemy's plans, accurately known, give the commander victories that look effortless. The general who fights without knowing the enemy's plans is fighting in the dark, however brave his troops.
The closing of the book returns to its opening: war is too costly to be undertaken lightly, and information is the cheapest way to ensure that the cost is not wasted. The strategist's first investment is always in seeing clearly. Everything else follows.
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