His academic career was interrupted by military service; he enlisted in the 18th Royal Fusilliers in 1914 and was commissioned in the 7th Oxford and Bucks Light Infantry in the following year.
He moved to Harvard University as Laura Spellman Rockefeller Fellow, 1925–26, but returned to Oxford as Deputy Professor of International Law and Diplomacy in 1927.
Hanbury (19 June 1898 – 12 March 1993), a fellow academic lawyer, describes Manning's attitude as follows: 'From 1964 onwards he was chairman of the South Africa Society, and was a brave apologist for his own country.
[1] This remark is expanded with the comment that 'Manning always insisted, with some passion, that scientific detachment [in his academic role] did not, and must not, mean refusal to commit oneself to causes in the political area, when laboratory coat and academic gown are doffed, and Manning did commit himself to at least one such cause, that of South Africa and its regime.
But scientific inquiry and political partisanship were at all times rigidly separated from each other in his mind, and only linked in so far as the political partisan, the committed voter in a democratic election or the professional politician, enact their chosen roles the better after serving their time as non-partisan students of the world in which their partisanship subsequently does its work.