Stephen Crane's "War is Kind" is an ironic poem. It means the opposite of what the title and the "war is kind" refrain say. What Crane is really communicating is that, although women (and men) are told a story that war is kind and virtuous, in reality war is cruel and horrible.
Crane uses the poetic devices of repetition and juxtaposition to emphasize the irony of saying war is kind. Like Antony in a famous speech in Julius Caesar, who conveyed that Brutus and the other men who assassinated Caesar were dishonorable by repeating that they were "honorable men" over and over, Crane ironizes "war is kind" by repeating it as an empty mantra. It's meaninglessness is highlighted by juxtaposing the phrase, along with "do not weep," against imagery that shows the actual horror of war. Imagery is another poetic device, in which a scene is described using the five senses of sight, sound, taste, smell, and touch.
Imagery, which shows rather than tells, is a powerful way of telling a story. Crane uses such images as
your lover threw wild hands toward the sky [as he died] [...]
A field where a thousand corpses lie
your father tumbled in the yellow trenches, Raged at his breast, gulped and died
juxtaposed against the empty statement "war is kind" to show that war is anything but kind.
The ironized juxtaposition occurs on a more granular level as well: the word "virtue" is placed near "slaughter," and "excellence" is placed near "killing." We read of the "splendid shroud of your son," and the words splendid and shroud create a jarring juxtaposition of a positive adjective and a negative image, for a shroud is a cloth that wraps around a dead body.
Crane also uses the poetic device of alliteration in the line "splendid shroud of your son," repeating the "s" sound three times at the beginning of words, which puts the emphasis on splendid, shroud, and son. This again highlights the irony: no parent of a son is likely to find his shroud splendid. Crane employs alliteration again in the ironic: "Great is the battle-god, great [...]" and in "weep" and "war" in the refrain.