What is Stephen Crane's message about war, and its impact on young men?

I think there are a few themes at play here. Certainly "manhood" is a theme that these young men have going into the war. Many brag of experiences on and off the battlefield that they never had. The young men quickly find that they substitute self-preservation for any romantic notions of war that they might have had. At one point Henry throws a pinecone at a squirrel and it takes off in fear. Henry believes that he has stumbled upon a universal truth: each being, animal or man, will do whatever it takes to preserve itself. This includes running away.