Upon the outbreak of World War II, Campbell returned to England and developed a close friendship with Welsh poet Dylan Thomas, which continued until Thomas's death. Also, despite being over-age and in very poor health, Campbell insisted upon enlisting in the British Army. While being trained for guerrilla warfare against the Imperial Japanese Army, Campbell was severely injured and ruled unfit for active service. After serving as a military censor and coast watcher in British East Africa, Campbell returned to the United Kingdom, where he befriended C.S. Lewis and J.R.R. Tolkien and briefly became a member of The Inklings.