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.