What is the exposition of "A Mystery of Heroism"?
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The exposition of this grimly realistic wartime tale plunges us into the middle of a brutal battle between the opposing sides of the Civil War. You might want to consider how even the opening sentence helps us to picture the scene:
The dark uniforms of the men were so coated with dust from the incessant wrestling of the two armies that the regiment almost seemed a part of the clay bank which shielded them from the shells.
Note how the two armies in an implied metaphor are compared to two wrestlers, fighting "incessantly" for dominance. Battle has been going on for so long that the soldiers are completely covered in dust and cannot be easily separated from the colour of the clay bank that they are hiding behind. The soldiers are hard pressed, and as we meet the story's protagonist, who is wishing he could have some water, the danger that they are under is emphasised by the death of the bugler by the "crimson terror of an exploding shell."
Thus the scene is set and the external conflict which will come to dominate the rest of the tale.