Which World War I battle did J.R.R. Tolkien experience as a soldier?

J.R.R. Tolkien experienced the Battle of the Somme during World War I.

Tolkien served as a second lieutenant in the Lancashire Fusiliers and went to France in 1916. He took part in the Somme offensive, one of the largest and bloodiest campaigns of the First World War. The battle began on 1 July 1916 and continued for months on the Western Front.

Tolkien became ill with trench fever and was sent back to England in October 1916. He spent much of the rest of the war recovering and performing duties in England. Most of the officers who had been his close school friends were killed, an experience that deeply affected him.

Readers sometimes claim that a single Middle-earth battle is a direct allegory for the Somme. Tolkien generally resisted simple one-to-one allegorical readings of his fiction. Nevertheless, his wartime experiences, loss, landscapes, and memories of industrialized conflict formed part of the historical background from which his imagination developed.