Contrast is a key element in this excellent short story, and Crane especially uses ironic contrast to highlight the absurdity and meaninglessness of war and yet how it has such a profound impact on individuals. A great place to start would be looking at the opening description, which presents us with the lieutenant, rather bizarrely, separating the company's coffee supply with his sword. Notice how the absurdity of the situation is highlighted by the description we are given:
He was on the verge of a great triumph in mathematics, and the corporals were thronging forward, each to reap a little square, when suddenly the lieutenant cried out and looked quickly at a man near him as if he suspected it was a case of personal assault.
The lieutenant, in the middle of battle, is on "the verge of a great triumph in mathematics" when he receives the wound that will necessitate the amputation of his arm. This is almost a ludicrous scene as the danger of war is contrasted with the trivial and rather meaningless task of dividing coffee supplies.
In addition, consider the final paragraph and the way in which the lieutenant tries to shrug off the importance of losing his arm in the face of the tears of his family. Contrast is a vital part of this story and is something that Crane uses to highlight his belief in the insignificance of man and our lives.