Doyle served as a volunteer doctor in the Langman Field Hospital at Bloemfontein between March and June 1900,[62] during the Second Boer War in South Africa (1899–1902). Later that same year, he wrote a book on the war, The Great Boer War, as well as a short work titled The War in South Africa: Its Cause and Conduct, in which he responded to critics of the United Kingdom's role in that war, and argued that its role was justified.