"An Occurrence at Owl Creek Bridge" (sometimes called "An Incident at Owl Creek Bridge") is a short story

Short story

A short story is a work of fiction that is usually written in prose, often in narrative format. This format tends to be more pointed than longer works of fiction, such as novellas and novels. Short story definitions based on length differ somewhat, even among professional writers, in part because...

 by Ambrose Bierce

Ambrose Bierce

Ambrose Gwinnett Bierce was an American editorialist, journalist, short story writer, fabulist and satirist...

. It was originally published in 1890, and first collected in Bierce's 1891 book Tales of Soldiers and Civilians. The story is famous for its irregular time sequence and twist ending.
Set during the American Civil War

American Civil War

The American Civil War was a civil war fought in the United States of America. In response to the election of Abraham Lincoln as President of the United States, 11 southern slave states declared their secession from the United States and formed the Confederate States of America ; the other 25...

, "An Occurrence at Owl Creek Bridge" is the story of Peyton Farquhar, a Confederate

Confederate States of America

The Confederate States of America was a government set up from 1861 to 1865 by 11 Southern slave states of the United States of America that had declared their secession from the U.S...

 sympathizer condemned to death by hanging

Hanging

Hanging is the lethal suspension of a person by a ligature. The Oxford English Dictionary states that hanging in this sense is "specifically to put to death by suspension by the neck", though it formerly also referred to crucifixion and death by impalement in which the body would remain...

 from Owl Creek Bridge.