Chapter IX

Sun Tzŭ classifies how to encamp on mountains, by rivers, in salt-marshes, and in dry country, then synthesizes general principles: high ground over low, sunny over dark, hard over damp. The four terrain rules are credited as the basis of the Yellow Emperor's victories.

Sun Tzŭ catalogs ground that traps an army (cliffs with torrents, sealed hollows, tangled thickets, quagmires) and tells the general to search every hilly neighborhood, pond, or thicket near camp for ambushes and spies.

Sun Tzŭ teaches inference from observed enemy behaviors: a quiet close enemy is relying on terrain; an aloof enemy wants you to advance; an easily-reached camp is bait. He extends this through visual cues — dust, birds in flight, peace proposals — each one a signal that betrays the enemy's intent.

Sun Tzŭ reads the inner state of an enemy army from outside: men leaning on spears are starving, soldiers who drink before drawing for the camp are thirsty, frequent rewards mean the general has lost his men. Even an angry march that doesn't engage is a sign — likely a feint preparing an ambush.

Sun Tzŭ closes the chapter on leadership: numerical parity is enough if you don't try to attack frontally; punishing soldiers before they trust you breeds insubordination; treating them with humanity then enforcing iron discipline produces an army that obeys because it believes.