The poet Stephen Crane is being intentionally ironic. He does not really believe that war is kind but that it is a horrible and senseless thing. The summary in the eNotes study guide for the poem states:
The title alerts us to the ironic tone of the poem, as it is very difficult to imagine war being kind in any way.
Crane contrasts the actuality of war with the glorified version which is meant to appeal to idealistic young men. He describes the uniformed men assembled in ranks with their drums and bugles and colorful flags, and then he juxtaposes this with descriptions of the battlefields strewn with corpses. He seems to be referring to the Civil War, in which the two opposing sides lost 620,000 men in battles. Hundreds of thousands more died of disease or in captivity. Crane's best known work is his novel The Red Badge of Courage, which is about a young man's experiences as an enlisted man in the Civil War. The Red Badge of Courage was first published in 1895. "War is Kind" was originally published in 1899.
Why does the poet keep repeating the refrain "War is kind"? In what sense can any war be regarded as "kind"? Crane is implying that the soldiers are relieved of all their fears and anguish when they die and that their loved ones are relieved of their concerns when they learn that their lovers, husbands, fathers and sons have been killed on the field of battle. The refrain "War is kind" is intentionally ironic. The poem is an expression of disgust with war, a feeling many Americans have experienced after the end of the Civil War, the Vietnam War, and other bloody engagements.