At the beginning of Glory, having never seen war before, Matthew Broderick's character, Robert Shaw, gets shell-shocked on the battlefield, unable to function with the reality of warfare exploding around him. In 1862, an educated man such as Shaw might have read books about war and seen pictures, but would have been almost completely surprised as to what actually occurs on the battlefield.
Today, potential soldiers have grown up watching increasingly realistic representations of war on TV, in full-color photography, at the movies, and in video games. Flat fictional representations of war still don't prepare people for what happens in combat, but it must be difficult to recruit someone who has seen the first 15 minutes of Saving Private Ryan or Full Metal Jacket (although some may sign up because of such films). Maybe the depiction of war in media has something to do with the anti-war protests as well; seeing war -- even watered-down on-screen combat -- is making people think hard about wanting to send anyone into that kind of situation, regardless of whether ousting dictators is a good idea or not.