What happened that makes the youth feel insignificant?

Henry's regiment is referred to as mule drivers and sent on a dangerous mission. The officer makes no bones about the fact that the regiment is expendable..... or insignificant. 

These happenings had occupied an incredibly short time, yet the youth felt that in them he had been made aged. New eyes were given to him. And the most startling thing was to learn suddenly that he was very insignificant. The officer spoke of the regiment as if he referred to a broom. Some part of the woods needed sweeping, perhaps, and he merely indicated a broom in a tone properly indifferent to its fate. It was war, no doubt, but it appeared strange.