This poem's imagery presents the horrors of war through the traumatized speaker's eyes. His memories of what he saw in a makeshift hospital one terrible night veer from foggy, overwhelmed impressions of hellish crowds to appallingly precise visions of glinting surgical instruments.

When he first arrives in the "dim-lighted" hospital, for instance, he sees a picture straight out of Dante's Inferno (which, not coincidentally, Whitman was reading at the time he wrote this poem). "Shadows of deepest, deepest black" are illuminated by "one great pitchy torch," a smoking beacon that emits "wild red flame and clouds of smoke." He remembers this sinister light in far more detail than he remembers what it illuminated: the "groups of forms" that lie groaning around the room appear only "vaguely" in the smoky light and in his memory. This movement from the specific to the vague suggests that much of what the speaker saw was just too terrible either to describe or to remember clearly; his traumatized mind blanks some of the specifics out, leaving only a nightmarish general impression behind.

That doesn't stop him from having nastily specific memories of what it was like in there: the "smell of ether" and the "odor of blood" return to him even as he describes the experience, as if he never really escaped. The "glisten of the little steel instruments" as desperate surgeons try to operate on dying men sticks with him, too.

By describing the room around the "crowd of the bloody forms" in more detail than he describes most of the dying or dead men, the speaker invites readers to imagine horrors beyond imagining and "beyond description."