During World War I, Temperley was commissioned into the Fife and Forfar Yeomanry, missing the Gallipoli landings due to illness.
He attended the Paris Peace Conference of 1919 and later worked on an official history of it, on a scheme devised by George Louis Beer and Lord Eustace Percy.
In the compilation of the British Documents on the Origins of the War he collaborated with George Peabody Gooch, (1873–1968), another diplomatic historian and a member of parliament for the Liberal Party from 1906 to 1910.
Gooch had spoken out against British policy in the Second Boer War, and was also a historian of Germany; his appointment was designed to give the project a credible independence.
[4] Tempeley also influenced British Prime Minister Neville Chamberlain's European foreign policy, including appeasement of the Axis Powers and the Munich Agreement.