There are certainly plenty of metaphors to choose from. You might want to think about the way in which Crane seems to liken men in combat to machines or animals, as in the death of the bugler that is referred to at the beginning of the story. Note how Crane does this, both in terms of the description of the men and in how the bugler dies:
As the eyes of half of the regiment swept in one machinelike movment, there was an instant's picture of a horse in a great convlusive leap of a death wound and a rider leaning back with a crooked arm and spread fingers before his face. On the ground was the crimson terror of an exploding shell, with fibres of flame that seemed like lances.
Note how the regiment are compared to robots with the implied metaphor of the movement of their eyes, which are described as moving in "one machinelike movement." Likewise, the picture of the bugler is depicted as not human, with a "crooked arm" dominating his appearance in the midst of the explosion. Also, we have the explosion of the shell compared to a "crimson terror."
Hope this helps to get you started. Now you can re-read the story and identify other metaphors that Crane uses. Good luck!