As with most young men, Henry Fleming wants to prove his manhood. And what better way to do that than seek glory on the field of battle? Neither Henry nor any of the other young men of his platoon have the faintest idea of what war actually involves. Their understanding of conflict—such as it is—is largely derived from storybooks and legends. War just seems like an awfully big adventure to them, a chance to show off your bravery and prove yourself as a man.
For Henry, war's not quite real; it's something he dreams about. This protects him from the harsh realities of war: the blood, death, pain, and suffering that it never fails to bring. The experience of war's so remote to Henry that it belongs in a bygone era of high castles and heavy crowns, a romantic age of chivalry and honor. Henry's mother had always tried to talk him out of joining up. But eventually Henry insisted, fired up by dreams of glory and egged-on by the talk of the village and the daily war reports in the newspapers.
In Stephen Crane's The Red Badge of Courage, protagonist Henry Fleming decided to enlist in the Civil War (in the Union Army) because he had a romantic view of warfare and desired to earn the glory reserved for great warriors. Having heard countless tales of the "Homeric" glory associated with warfare, Henry desired to prove himself a hero (like Odysseus, Achilles, Hector, and the many other heroes whom Homer wrote about).
Despite this, Henry feared he would not have the courage to face the battle. As the narrator says:
Whatever he had learned of himself was here of no avail. He was an unknown quantity.
Having never experienced anything quite like combat, Henry did not know whether he had the courage necessary to attain the glory he so desired. These fears caused Henry (and cause the reader) to reconsider his presuppositions about the relationship between glory and courage--and whether or not cowards could earn glory.