At the beginning of "How to Tell a True War Story," Rat Kiley writes to the sister of his friend, Curt Lemon, who has just been killed in Vietnam. Kiley writes at length about what a fantastic friend and what a brave man Lemon was, pouring his heart and soul into the letter. He never receives a reply. After relating this, Tim O'Brien observes,

A true war story is never moral. It does not instruct, nor encourage virtue, nor suggest models of proper human behavior, nor restrain men from doing the things men have always done. If a story seems moral, do not believe it.

Later in the story, the soldiers find a baby water buffalo. Rat Kiley first strokes its nose and offers it some of his rations, but the water buffalo is uninterested. Kiley then starts shooting at the buffalo. He does not shoot to kill, but to hurt, aiming first at the animal's knee, then taking off an ear. Kiley shoots the animal again and again, finally taking aim at the nose and the throat. It takes a long time to die and is still not quite dead when the other soldiers pick it up to dump it in the village well.

The oppressive atmosphere in Vietnam, the death of his closest friend, and the indifference of others to that death cause Kiley to snap and take out his aggression on the baby water buffalo. The other soldiers seem to understand his feelings, as O'Brien relates that

The whole platoon stood there watching, feeling all kinds of things, but there wasn't a great deal of pity for the baby water buffalo. Curt Lemon was dead. Rat Kiley had lost his best friend in the world. Later in the week he would write a long personal letter to the guy's sister, who would not write back, but for now it was a question of pain.

This moment ties back to the aforementioned quotation about the nature of a true war story. Rat Kiley's killing of the buffalo and the platoon's ambivalence about that cruel act are surprising and morally ambiguous, if not questionable. But that quality of ambiguousness—of messiness and discomfort—allows it to ring true as a war story.