He served with the Cameronians (Scottish Rifles) in two World Wars, rising to the rank of brigadier, and after retiring from the army in 1946 was editor of The Army Quarterly and Defence Journal from 1950 to 1966, co-editor of Brassey's Annual: The Armed Forces Year Book from 1950 to 1969, and contributed articles to encyclopedias and periodicals.

[2] Barclay was commissioned in the Cameronians (Scottish Rifles) in January 1916[4] and served in France and Mesopotamia during the First World War.

[2] He visited the United States twice in 1943 in the course of his duties, describing himself as Scottish but born in Dartford, Kent, and able to read and write English, Arabic, and Hindustani.

[11] The book was published in London by William Clowes and Sons in 1953, in New York by The Philosophical Library, and reprinted by Greenwood Press in the United States in 1983.

[12] In 1964, The Economist praised Barclay's study of British military leadership in the early part of the Second World War, On their Shoulders.

The magazine felt that Barclay's careful judgements, which contrasted with some less objective works, avoided the lionisation of the successful and the damning of those who failed, and in his comments on British Army training deficiencies, helped to explain some of the early military failures during the war.

[16] The dust jacket features an image of the fighting while the front boards reproduce a line drawing of The Cenotaph in London's Whitehall with draped flags.