What is the underlying theme of The Sniper?

The key underlying theme O'Flaherty exposes in "The Sniper" is the absolute futility, and unnaturalness, of civil wars such as the one being fought in Ireland at the time the story is set. Wars of this sort create such division in families that the two brothers in the story are fighting on opposite sides; the protagonist inadvertently kills his brother, without ever knowing it was him, because their identities as Republicans and Unionists have depersonalized them so entirely. He does not kill his brother by accident—on the contrary, he kills him deliberately, because of what he is in political terms. He may not know that this is his brother in literal terms, but he certainly knows that he is, in the broader sense, his brother, a fellow Irishman. The outcome of the story, a brother killing his own flesh and blood, is a metaphor for the broader war as a whole. This type of civil conflict drives rifts between families, but moreover drives an unnatural rift between Irishmen based upon their political alliances.