Henry had grown up with a certain image of combat and an idea about the way it could act as a proving ground.  He was nervous about it, but in some ways he may have been looking forward to it as it gave him a chance to prove his mettle under fire.  If you look closely at his progression throughout the book, each time he does approach it with trepidation, but he comes away from it with something to build on, even when he actually runs away.  in that incident, he still gets some respect form his peers because they think he's been wounded so it has been reaffirmed in his mind that combat is a place to prove oneself and to gain respect.  As such it is something to be looked forward to despite the horror of it.