Both J.R.R. Tolkien and C.S. Lewis served in the First World War. Chris Gehrz has written on this:
Tolkien, older than his future friend by nearly seven years, entered military service first, arriving in France in June 1916 — not quite three months after getting married and mere weeks before the beginning of the Battle of the Somme. “Junior officers were being killed, a dozen a minute,” he remembered. “Parting from my wife then… it was like death.” And while he suffered a severe case of the lice-born ailment known as “trench fever,” Tolkien survived the war. All but one of his best friends, he later recalled, did not.
You can read the full article here.