火攻
A technical chapter on fire attacks that closes with the treatise's gravest warning: act only on advantage, never on anger — moods pass, but a destroyed state and the dead never return.
What this chapter says
On the surface this is a practical chapter on the five ways to attack with fire and the conditions for using it — the right seasons, dry weather, and favourable wind — together with how to follow up a fire attack and never let it burn without exploiting it. But the chapter's lasting importance lies in its closing passage, which rises far above fire tactics into the moral core of the whole treatise. A ruler must never mobilise an army merely to gratify his anger, nor a general give battle out of pique. Anger can in time turn back to gladness, and vexation to content — but a kingdom once destroyed can never exist again, and the dead can never be brought back to life. Therefore the enlightened ruler is prudent and the good general full of caution. It is Sun Tzu's clearest statement that strategy must be governed by cold calculation of interest, never by passion.
Key lines
Applied today
Never make a major, irreversible move — a lawsuit, an acquisition, a mass layoff, a public feud — out of anger or wounded pride. The mood will pass in a week; some decisions can never be undone. Sleep on anything you can't reverse.
Act on clear advantage and real gain, not on the heat of the moment. Feelings are poor guides for choices that can't be reversed — check the interest, not the impulse, before you commit.