There were many things that C.S. Lewis and J.R.R. Tolkien had in common, including a love of literature, brilliant minds, gifted writing and Christian faith. One of the other things that they had in common is that they both served in World War One.
Paul Glader interviews Joseph Loconte, author of A Hobbit, a Wardrobe, and a Great War: How J.R.R. Tolkien and C.S. Lewis Rediscovered Faith, Friendship, and Heroism in the Cataclysm of 1914-1918, for American Legion magazine. You can find the article here.