On the outbreak of the Greco-Turkish War in April 1897, Callwell was attached to the Greek army and spent a year in the Near East. In October 1899, when war was declared against the Boer Republics in South Africa, Callwell was appointed to the staff of Sir Redvers Buller, and was present throughout the operations which ended with the relief of Ladysmith on 28 February 1900. In September 1901 he received a mention in despatches from Earl Roberts, and was awarded the brevet rank of lieutenant-colonel (dated to 29 November 1900), and given command of a mobile column, with which he served in the Western Transvaal and in Cape Colony until the close of the war in June 1902. He left Cape Town for England the following month, and arrived in Southampton in August 1902.