At the outset of the American Civil War, [the writer Ambrose] Bierce enlisted in the Union Army's 9th Indiana Infantry Regiment....In February 1862 he was commissioned First Lieutenant, and served on the staff of General William Babcock Hazen as a topographical engineer, making maps of likely battlefields.

Bierce fought at the Battle of Shiloh (April 1862), a terrifying experience that became a source for several later short stories and the memoir, "What I Saw of Shiloh". In June 1864, he sustained a serious head wound at the Battle of Kennesaw Mountain, and spent the rest of the summer on furlough, returning to active duty in September. He was discharged from the army in January 1865. (Summary by Wikipedia)