He was commissioned as a lieutenant in July 1878,[2] joining a battery of the 3rd Brigade, Royal Field Artillery, then stationed in India, and serving in the closing stages of the Second Anglo-Afghan War.
[6] This was later expanded into a book Small Wars: Their Principles and Practices, published in 1896, which was adopted as an official British Army textbook, and won wide recognition.
[8] The United States Marine Corps Small Wars Manual, originally published in 1935, drew heavily on Callwell's book,[9] and as the first comprehensive study of what came to be known as "asymmetric warfare", it gained renewed popularity in the 1990s,[10] and remains in print.
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.
[1] In September 1901 he received a mention in despatches from Earl Roberts,[20] and was awarded the brevet rank of lieutenant-colonel (dated to 29 November 1900),[21] 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.
[22] A year after his return to England, he was appointed a Deputy Assistant Quartermaster-General in the mobilization branch of the War Office on 6 October 1903,[23][1] and by April 1904 was working in Intelligence once again.
[27] Having seen several of his contemporaries promoted to general officer rank over his head,[1] Callwell eventually quit the army in June 1909,[28] to devote himself to writing.
[1] In December 1915, following on the appointment of Sir William Robertson as Chief of the Imperial General Staff, a reorganization took place at the War Office.