The short story "The Sniper" was written by Liam O'Flaherty and takes place in Dublin during the Irish Civil War. It tells the story of a Republican sniper stationed on a Dublin rooftop across from a Free State (or National Army) sniper, who shoots him in the arm. The Republican sniper eventually shoots and kills the enemy gunman, but it is only at the very end of the story that he realizes, along with the reader, that he has shot his own brother.
The text gives us several clues that the two brothers belong to opposing armies. For example, O'Flaherty tells us right in the first paragraph that the Republican sniper is fighting in the Irish Civil War, which shows us that he must be aiming at targets from the opposing side: "Republicans and Free Staters were waging civil war."
In the next paragraph, we then find out which side the sniper is on, as the author tells us that "a Republican sniper lay watching." When we first encounter the other man, who later turns out to be the first sniper's brother, we hear that the Republican sniper considers this man to be an enemy: "He must kill that enemy." This clearly indicates that the other man is fighting for the other side. The fact that the other gunman shoots first at the sniper also indicates that these two men must be fighting for different armies.