Stephen Crane is rightfully famous for his ability to write stories that are excellent examples of Realism, not Romanticism. Realism tries to present life as it actually is, without glossing over any of the nasty bits. As the majority of Crane's fiction is set during the Civil War, this means that his narratives include lots of gory descriptions of fighting, injuries and blood. For examples of this, you might want to focus on the description of the Field Hospital that the Lieutenant reaches at the end, and the way in which there are "interminable" numbers of wounded and groans can be heard. Note the following incident:
Sitting with his back against a tree a man with a face as grey as a new army blanket was serenely smoking a corn-cob pipe. The lieutenant wished to rush forward and inform him that he was dying.
The details of the man's face, which is compared to a "new army blanket," and the way in which he is "serenely" smoking add an element of realism to the account, as he is obviously patiently waiting his own death by himself. Note too the way in which the lieutenant is treated by the surgeon when the surgeon realises he needs to have his arm amputated:
Then he caught sight of the lieutenant's arm and his face at once changed. "Well, let's have a look at it." He seemed possessed suddenly of a great contempt for the lieutenant. This wound evidently placed the latter on a very low social plane.
The lieutenant, in spite of his bravery, is shown to be treated in a terrible way by the surgeon, who feels the lieutenant now occupies "a very low social plane" because of his injury. Such notes of realism act as an antidote to Romantic notions of war concerning heroism and bravery. Here, the lieutenant doesn't even face an enemy, and is just shot from behind the screen of trees. His amputation and the effect this has on his life is described as being just another "Episode of War" that explores the true horror of warfare and does not present war as Romantic in any way.