When scholars talk about Stephen Crane, they love to use the word "realism." Essentially, realism means exactly what you've probably guessed it does: making things look like they would in real life. Crane worked as a journalist for quite some time, so he certainly had a flare for describing things as they actually occurred.
Crane's most famous novel (The Red Badge of Courage), for example, is full of realistic battle scenes that very closely resemble actual Civil War battles. In addition to "getting it right" when it comes to the battles themselves, Crane's novel also offers a realistic depiction of its main character's complex emotional struggles.
In "War is Kind" we see the same attention to detail. Did you notice, for example, all those gory details about fields with a thousand corpses, guys throwing their hands to sky, and guys falling into trenches and dying? War is ugly and full of death, just like this poem. For reals.
Besides all that stuff, consider the speaker's whole mentality and manner of speaking. They very closely resemble those of an actual military leader of some kind. Add to that the fact that the poem really sounds like war. Battlefield noises are here in all their glory ("booming" and "blazing," 6, 17), as are the sergeant's commands: "Point," "Make plain" (20, 21).