Chapter 30

The chapter is advice to a military adviser rather than to a general, and it opens by defining the job negatively: one who assists a ruler with the way does not coerce the world with arms.

The reason offered first is not moral but mechanical, that such things rebound. Then comes the passage the chapter is remembered for. Where armies have camped, brambles grow. After a great campaign there is sure to be a bad harvest.

Both are agricultural observations and both are about what happens after the fighting, on ground nobody is fighting over any more. An army in the field eats the crop, consumes the labour that would have planted the next one, and leaves land that reverts to scrub because there is nobody to work it. The cost the chapter names is not casualties and not defeat; it is the two or three years of hunger that follow a victory, which is a cost a ruler can inflict on his own population by winning.

The rest of the chapter concedes that war sometimes happens and sets a standard for it. A good general achieves the result and stops. Then five clauses, all negative: achieves it without boasting, without vaunting, without pride, because there was no choice, and without coercion. The fourth of those is the constraint that matters, since it makes necessity the only licence.

The closing line is one of the book's recurring formulae, that things which grow strong then grow old, and that this is contrary to the way and ends early. Applied here it says that a state which has become powerful by arms is on a schedule.

Four chapters in this book are about the conduct of war: thirty, thirty-one, sixty-eight and sixty-nine. Read together they form a small and consistent body of doctrine, and they are the strongest evidence that the text belongs to the strategic literature of its period as well as to its philosophical literature.

The doctrine is roughly this. War is an instrument of last resort and its costs outlast the campaign. A good general stops when the object is achieved and does not convert victory into seizure. Anger and aggression in a commander are technical faults rather than moral ones, because they produce bad decisions. The defensive posture is stronger than the offensive one, and the side that grieves for the fighting is the side that wins.

That last claim, from chapter sixty-nine, is the one that shows the chapters are not pacifist. They assume there is a battle, they assume somebody wins it, and they say which side does. Chapter sixty-eight ranks kinds of competence in a warrior. Chapter thirty accepts that a campaign may be undertaken because there was no choice.

The vocabulary here overlaps with the military writing of the Warring States, which also prized calculation over ardour, treated the enemy's momentum as something to be borrowed rather than met, and regarded a war won without battle as the best outcome. What this book adds is chapter thirty-one, which is not about effectiveness at all: it requires that a victory be conducted with funeral rites, and no manual of strategy asks that.