Why did the luster fade from the eyes of the men in the 304th?

The answer to this question, quite simply, is that the enemy was closing quickly and ferociously upon the 304th. Crane describes the scene as such:

"The men groaned. The luster faded from their eyes. Their smudged countenances now expressed a profound dejection. They moved their stiffened bodies slowly, and watched in sullen mood the frantic approach of the enemy. The slaves toiling in the temple of this god began to feel rebellion at his harsh tasks."