Poole served in the Life Guards for nine months, and was commissioned as an officer in the Warwickshire Yeomanry in 1934. During the Second World War, he fought in Syria, Iraq, North Africa, Italy and North-West Europe, rising to become a colonel in the British Army on the staff of the 21st Army Group. He was thrice mentioned in despatches. He was appointed to the Order of the British Empire as a Member (military) on 18 February 1943, and was promoted to be an Officer (military) on 16 September that year, and Commander (military) on 11 October 1945. He was also appointed to the American Legion of Merit as a Legionnaire "in recognition of distinguished services in the cause of the Allies" on 15 March 1945, and to the Dutch Order of Orange-Nassau with Swords (denoting the military division) as a Commander on 16 January 1947.