Crane's story is, like The Red Badge of Courage, a series of snapshots, so to speak, of the enormous conflict that was our Civil War. We are shown the action from a localized viewpoint, in which the enlisted men and the lower-ranking field officers who command them do not themselves see the big picture, but can only follow orders and guess at what the tactics and the strategy of the commanders are. When the lieutenant is wounded and goes to the rear of the line for treatment, he catches glimpses of the generals, the delivery of messages that involve the battle plan, the movements of the big guns, and all is like "an historical painting." The lieutenant's vision of this symbolizes the fact that soldiers and combat officers alike have no direct idea of what is happening on the highest level of war, or why it's happening. Each man, and each unit, are just a tiny piece of a huge, incomprehensible event.
The usual estimate by historians is that approximately 620,000 men died in the Civil War. This includes deaths from disease as well as directly in battle. More recently, some historians have speculated that the total is even higher. In Crane's story, the focus is on one man, the lieutenant, and a bullet to his wrist which necessitates the amputation of his arm. That we are not shown details of the massive carnage that took place at this particular battle, and the number of men killed, is in keeping with the selective, individual viewpoint that Crane typically gives us, which is horrific enough in itself.