Which British Army regiment did J.R.R. Tolkien serve with during the First World War?

During the First World War, J.R.R. Tolkien served with the Lancashire Fusiliers.

Tolkien was commissioned as a temporary second lieutenant in 1915 after studying at Oxford. He joined the 11th (Service) Battalion of the Lancashire Fusiliers and went to France in 1916. He took part in the Battle of the Somme, one of the war's largest and bloodiest campaigns.

Tolkien later suffered from trench fever and was sent back to England to recover. Nearly all of the officers in his battalion were killed during the Somme fighting, an experience that shaped his understanding of loss, loyalty, and fellowship. Claims that Tolkien served in a different regiment usually confuse his military unit with other British formations active on the Western Front.