The answer to your question can be found at the beginning of the story, in the first paragraph. One of the key aspects that it is important to realise at the beginning of this excellent tale is the constant danger that the soldiers are in. This is because of the way that the opposition forces have a battery on top of the hill, giving them a real tactical advantage as the battery is able to fire over enemy fortifications and target soldiers as it likes because of its superior vantage point. Consider what the text says:

On teh top of the hill a battery was arguing in tremendous roars with some other guns, and to the eye of the infantry, the artillerymen, the guns, the caissons, the horses, were distinctly outlined upon the blue sky.

Notice how the good positioning of the battery is emphasised by the way that we can picture the soldiers looking up at it and seeing how it was "distinctly outlined" against the blue sky.