Do not attack from the leeward. A wind that rises in the daytime lasts long, but a night breeze soon falls. This is what happens as a general rule. In every army, the five developments connected with fire must be known, the movements of the stars calculated, and a watch kept for the proper days. By means of water, an enemy may be intercepted, but not robbed of all his belongings.
Water can do useful service, but it lacks the terrible destructive power of fire.
This is the reason, Chang Yu concludes, why the former is dismissed in a couple of sentences, whereas the attack by fire is discussed in detail. Here I feel convinced that it is an interpolation, for it is evident that ss. 20 ought to follow immediately on ss. Hence the enlightened ruler is heedful, and the good general full of caution. This is the way to keep a country at peace and an army intact.