The setting of "An Occurrence at Owl Creek Bridge" is the Southern United States, during the U.S. Civil War. This allows a clear delineation between two sides, allowing the reader to choose sympathies. The titular bridge is one that serves a railroad track, allowing Union soldiers better access to strategic points. When Payton Fahrquhar, a Confederate sympathizer, tries to sabotage the bridge, he is caught and sentenced to be hanged off the bridge itself as an example.
A man stood upon a railroad bridge in northern Alabama, looking down into the swift water twenty feet below.
A piece of dancing driftwood caught his attention and his eyes followed it down the current. How slowly it appeared to move! What a sluggish stream!
The first sections of the story are mild and slow, contrasting the relaxed atmosphere of the American South with the war effort waged across it. The "sluggish" stream is in counterpoint to the trains that cross the bridge, bringing rushing activity and death.
At the end, Payton is suddenly free to escape, and enters the woods around his plantation:
The forest seemed interminable; nowhere did he discover a break in it, not even a woodman's road. He had not known that he lived in so wild a region. There was something uncanny in the revelation.
(Quotes: Bierce, "An Occurrence at Owl Creek Bridge," eNotes eText)
Although this is all a dying dream, Payton can be said to feel the strangeness and isolation of his homeland, one he thought he knew but now finds alien. Before his attempted sabotage, he lived in peace, with only the news of the war and little interference; his hubris in trying to help instead of standing back brought him to his doom, and the bridge -- which acts as both a literal bridge and the "bridge" between life and death -- destroys him as completely as he wished to destroy it.