Which war did J.R.R. Tolkien serve in as a second lieutenant?

J.R.R. Tolkien served in the First World War as a second lieutenant.

After graduating from Oxford and marrying Edith Bratt in 1916, Tolkien joined the British Army. He was commissioned into the Lancashire Fusiliers and served on the Western Front during the First World War.

Tolkien took part in the Battle of the Somme in 1916. He later became ill with trench fever and was sent back to England for treatment. Many of the officers and friends he knew during the war were killed, and the experience left a lasting impression on him.

It is too simple to claim that The Lord of the Rings is a direct allegory of the First World War. Tolkien rejected that kind of one-to-one interpretation. Nevertheless, wartime loss, mechanized destruction, companionship, and recovery form important parts of the historical background surrounding his writing.