Stephen Crane's The Red Badge of Courage is more an "important" book than a "great" book (we all know plenty of these). I don't think I would classify it among the highest echelon of literature. Crane was twenty-four when he wrote it, had no military experience, and wrote in a unique style that some can't stand and some consider laden with thematic depth. And yet Crane was covering some new ground. In the mid-1890s he was straddling the divide between romanticism and modernism, between the plot-driven, externally-focused tradition of the war narrative and the more contemporary psychological study of the individual soldier.
His Henry Fleming (or, frequently, "the youth") is in many ways both protagonist and setting for the novel, so much of the action taking place inside his head. He wants to be a hero, to do grand things, but he's a bit of a coward. The book explores this conundrum through Henry's ongoing conflict to reconcile his ambition and sense of duty with his fear. It's a tension I suspect isn't uncommon among those uniform, though like Crane I've never served in uniform and can't claim to know. We all like to think we'd be brave, but that mastery of fear and willingness to die for something theoretically greater than ourselves is something no man or woman can know until in that situation.
There are other themes that pervade Fleming's narrative as well, questions of proving our worth in war, of honesty and stolen valor, or the fleeting nature of life, of the cost of war. It's a dense novel, for all that so little actually happens in it. But it is one of the earliest fictional attempts to understand the psyche of the solider, the razor's edge between heroism and cowardice, and the blending of the truths and myths of war.