Precisely because Crane is showing us that the real battle is not a physical, exterior affair, but an internal matter in the mind of his protagonist, Henry, as he has to struggle with his own understanding of war, bravery courage and match it to the reality that he sees and experiences, acknowledging his very real emotions of fear and cowardice. Note Henry's first response to being told that he might face war the next day:
The youth was in a little trance of astonishment. So they were at last going to fight. On the morrow, perhaps, there would be a battle, and he would be in it. For a time he was obliged to labour to make himself believe. He could not accept with assurance an omen that he was about to mingle in one of those great affairs of the earth.
To Henry, then, war is "one of those great affairs of the earth," and we are told that war had been part of his intense imagination when growing up, with battles being regarded as "crimson blothes on the pages of the past." However, the reality of facing a battle the very next day forces Henry to seriously question his own understanding of such morally abstruse concepts such as bravery and courage, and the lengthy flashback in the first chapter helps establish the discrepancy between his thoughts about war and the reality of war. His question to Jim as to whether he might "run" and desert the battlefield indicates his own deep sense of fear and cowardice at what is about to happen.
Thus the length of time it takes Henry to contemplate his part in the battle serves to introduce the conflict that drives this novel, as we, like Henry, have to question our own notions of courage and bravery.