"What I Saw of Shiloh" is Bierce's account of the bloodiest day of the Civil War for the Federal forces up to 1862, and although Bierce was an officer in the topographical corps (map-makers), he would have seen the disaster unfold and its aftermath. In the story, we see the action through the eyes of an infantry platoon commander.
One of Bierce's techniques in all of his Civil War stories is to precisely and carefully describe a scene—often a scene of horror—and then have the scene's narrator, in frustration, claim that he cannot adequately describe the horror by which he is surrounded. For example, as he moves his troops up to support another unit that is facing the enemy, he notes, with some surprise:
Then—I can’t describe it—the forest seemed all at once to flame up and disappear with a crash like that of a great wave upon the beach—a crash that expired in hot hissings, and the sickening “spat” of lead against flesh. A dozen of my brave fellows tumbled over like ten-pins.
Well, despite his protestations that he cannot describe the scene, he has done a masterful job of describing the forest exploding with "hot hissings" of bullets and the sound of those bullets hitting the flesh of his men. Even then, the effects are so horrific that the narrator believes his words are inadequate to accurately convey the horror of the scene. The problem is not that he cannot describe the scene but that such scenes cannot be described in such a way that conveys the full horror of the event.
Later, after the battle, a fire sweeps the field and consumes everything, including the men who have fallen. As the narrator observes the effects of the fire, he again paints a picture:
According to degree of exposure, their faces were bloated and black or yellow and shrunken. The contraction of muscles which had given them claws for hands had cursed each countenance with a hideous grin. Faugh! I cannot catalogue the charms of these gallant gentlemen who had got what they enlisted for.
Bierce's narrator, despite his exclamation that he cannot describe the scene, has succeeded in depicting the horrific results of the fire on the corpses. Still, as with his earlier frustration, he feels instinctively that the full horror is beyond description. By this point, however, the narrator is able to bring some irony and detachment to the scene; "the charms of these gallant gentlemen" indicates that the narrator is so overwhelmed by the horror before him that he shields his sanity by exercising irony.
In his short story "Chickamauga," also a battle that Bierce would have participated in, the main character is a young boy who, as we discover at the end of the story, is a deaf mute. After he wanders away from his home looking for firewood, he encounters the battlefield, including the dead and the dying. He doesn't seem to understand the full import of what surrounds him—primarily because the scene is so foreign to him that it's like a dream, unreal and unrecognizable.
As the child approaches a burning building, he realizes that he is looking at his home, and he is "stupefied by the power of revelation." The horror for him, unfortunately, is only beginning as he sees a bloodied corpse lying face up on the ground. As he recognizes his mother, the child makes the only sounds he can:
He uttered a series of inarticulate and indescribable cries—something between the chattering of an ape and the gobbling of a turkey—a startling, soulless, unholy sound, the language of a devil. The child was a deaf mute.
Bierce's characterization of the sounds as "soulless, unholy sound, the language of a devil" is not meant to characterize the child but to describe the language of horror, the language that war engenders in even those who cannot articulate their horror in words. And, yet, the inarticulate sounds of the child are as effective as words.
Bierce, as good a wordsmith as we find, understands that the horror of war can be described but that no description can adequately convey war's full effects on the observer's psyche. In an ironic way, the limitations of language lie in the words themselves.