At the outset of the Second World War, broadcasting was recognised as part of the war effort. Hargreaves' talents in this field meant that he faced being recruited to a restricted post in radio, a reserved occupation. Instead, he joined the Royal Artillery as a private, quickly became an NCO, entered the Royal Military College, Sandhurst, and was commissioned into the Royal Tank Regiment. Even so, Hargreaves' reputation as a communicator went ahead of him. He was recruited to the staff of General Montgomery to play a role setting up broadcasting services to allied forces before and after D-Day. He left the army in 1945 with the substantive rank of major, having briefly held the acting rank of lieutenant-colonel.