Tim O'Brien's "How to Tell a True War Story," narrated by a character we identify with O'Brien himself because we know his biography, has several protagonists--each member of the platoon can be viewed as a protagonist, with the war itself being the antagonist.
In order to determine what effect war has on the narrator, we need to analyze what he says about war stories and, by extension, war itself. For example, the narrator characterizes a true war story as
. . . never moral. It does not instruct, nor encourage virtue, nor suggests models of personal behavior, nor restrain me from doing things they have always done.
If, as the narrator points out, true war stores have an "allegiance to obscenity and evil," it is difficult not to conclude that he is also describing war itself.
Perhaps the clearest and most horrible effect of war on men is in the episode in which Rat Kiley, mourning the loss of his friend Curt Lemon, tortures a baby buffalo, and after it has been shot to pieces, literally, other troops drop it, perhaps still alive, into the village well in order to ruin the water supply. The most telling part of that episode, ironically, is not what Rat Kiley did, but what the rest of the platoon failed to do--to stop him. The other characters, which, by the way, includes the narrator, are puzzled, but not horrified, by the scene:
The rest of us stood in a ragged circle around the baby buffalo. . . . We had witnessed something essential, something brand-new and profound, a piece of the world so startling that there was not yet a name for it.
War's effect on the narrator and others in the platoon is to create men who no longer function with what used to be called a moral compass. No one views Rat Kiley's actions as either appropriate or inappropriate because these troops live in a world in which morality and immorality are not relevant philosophical constants as they are (generally) in civilian life. In other words, the narrator is illustrating his definition of a true war story with a war story, and he has already told us that true war stories are relentlessly ugly and have no point. By extension, then, war itself--at least in the narrator's view--is unredeeming, ugly, and pointless.
At the end of the story, the narrator describes both war stories and war when he says
It's about love and memory. It's about sorrow. It's about sisters who never write back and people who never listen.