"An Occurrence at Owl Creek Bridge" is not a true story. The story's author, Ambrose Bierce, did fight on the side of the Union army in the American Civil War, participating in several well-known battles. It is interesting to note that he does not seem to revel in the execution of Peyton Farquhar—a Confederate sympathizer and saboteur—in this story. Bierce humanizes Farquhar rather than demonizing him and shows him to be a loving husband and father.
In his real-world experiences during the Civil War, Bierce witnessed a great deal of bloodshed. He also won recognition for saving a badly wounded friend during battle and even sustained a dangerous brain injury himself. But his treatment of the fictional Farquhar does not seem to indicate that it caused him to become bitter or to hate those individuals against whom he fought. If anything, the story seems to suggest that the tragedy of war itself reaches on and on, into spaces we cannot completely know or understand.
Instead of demonizing Farquhar or the Confederate cause, Bierce explores death in this story—something anyone will experience, regardless of where their loyalties lie. Bierce presents death as a formal affair, with the narrator describing it as
a dignitary who when he comes announced is to be received with formal manifestations of respect, even by those most familiar with him.
Similarly, Farquhar, in a ceremoniously solemn way, fixes his thoughts on what is most important to him in his last moment: his family and children (or, at least, he attempts to—in reality, he imagines escaping death and returning to them). Bierce highlights, too, the various sensations someone being executed might experience, from dread and delirium to the sharp pain of a snapping neck.
Thus, while "An Occurrence at Owl Creek Bridge" is not a true story, Bierce certainly gives the story a sense of realism through his believable depictions of death.