The last line of the chapter is one of the strangest sentences in the book. When two forces meet and are matched, the one that grieves for it wins.
Read as tactics it makes a defensible claim. An army that regards the fight as a disaster is an army that has not talked itself into it, that has exhausted the alternatives, and that is fighting because it must, which is the condition chapter thirty sets when it says a good general acts because there was no choice. Troops in that position are defending something, and the side that has convinced itself the war is glorious is the side more likely to have miscalculated.
Read against chapter thirty-one it is more than tactics. That chapter requires a victory to be conducted with funeral rites and says a person who delights in killing will not get his way in the world. This chapter says the same thing as a prediction about outcomes: grief is not merely the proper response afterwards, it is the disposition that decides the engagement.
The two readings are not rivals, and the book habitually offers a moral position in the form of a prediction. What it almost never does is say that a good outcome is deserved. It says the violent do not die a natural death, that a thing which grows strong grows old, and here that the mourning side wins, and each is stated as how things go rather than as how they should.
The specific verb has also been read as compassion rather than grief, which gives the line as its subject the side that pities, and connects it directly to the first of the three treasures.