What was the name of the battle in which Tolkien served as a signals officer in 1916?

Tolkien served as a signals officer during the Battle of the Somme in 1916.

After completing his studies at Oxford, Tolkien joined the British Army during the First World War. He was commissioned in the Lancashire Fusiliers and trained as a signals officer. He went to France in June 1916 and took part in the Somme offensive, one of the war’s largest and bloodiest campaigns.

Tolkien became ill with trench fever and was sent back to England in October 1916. The illness kept him from returning to the front, and most of the members of his close group of school friends, which he called the TCBS, were killed in the war. His wartime experience strongly influenced his feelings about friendship, loss, industrial destruction, and the endurance of ordinary people.

Tolkien did not fight at Verdun, Gallipoli, or Passchendaele. Passchendaele occurred in 1917, after his active service at the front had ended.