Final answer:
Ambrose Bierce, a short story writer and journalist, enlisted in the Civil War and wrote about the futility of war. Ernest Hemingway, often confused in this context, served during World War I and wrote about its brutalities in 'A Farewell to Arms' and other works. The correct answer to which writer enlisted in the Civil War and wrote about its ironic futility is Ambrose Bierce.
Step-by-step explanation:
The writer you're asking about, who enlisted in the Civil War and wrote about the ironic futility of war in his stories, is Ambrose Bierce. Bierce is a short story writer and journalist known particularly for his realistic narratives of the Civil War, providing a stark depiction of the brutal realities of war.
His work includes the famous collection titled "Tales of Soldiers and Civilians" which contains the story "What I Saw of Shiloh" (1881), a vivid account of his experiences at the Battle of Shiloh.
While Ernest Hemingway did serve in a war, he worked as an ambulance driver during World War I, not the Civil War, and he too wrote about the brutalities and ironies of war but his experience came from a different era. This is a common confusion because both writers dealt with war in their writings but from different conflicts and different perspectives.