During the First World War (in 1915), Rathbone was called up via the Derby Scheme into the British Army as a private with the London Scottish Regiment, joining a regiment that also included his future professional acting contemporaries Claude Rains, Herbert Marshall and Ronald Colman at different points through the conflict. After basic training with the London Scots in early 1916, he received a commission as a lieutenant in the 2/10th Battalion of the King's Liverpool Regiment (Liverpool Scottish), where he served as an intelligence officer and attained the rank of captain. Rathbone was twice the British Army Fencing Champion, a skill that served him well in movies and allowed him to even teach actors Errol Flynn and Tyrone Power swordsmanship. Rathbone's younger brother John was killed in action on 4 June 1918."Famous 1914–18," Richard Van Emden, 2010, p. 132 In 2012, two letters Rathbone wrote to his family while serving on the Western Front were published. One reveals the anguish and anger he felt following the death of his brother, John:
