He was a 21-year-old bank clerk in Dulwich when the First World War broke out; in September 1914 he voluntarily enlisted (under the name Leslie Howard Steiner) as a Private with the British Army's Inns of Court Officer Training Corps in London. In February 1915 he received a commission as a subaltern with the 3/1st Northamptonshire Yeomanry Regiment, with which he trained in England until 19 May 1916, when he resigned his commission and was medically discharged from the British Army with neurasthenia.