He served with the British Army in the Norwegian campaign during the Second World War, which began in September 1939, where he was one of the first British officers to engage the enemy. His work in blowing up bridges as the British retreated from Lillehammer earned Jackson his first Military Cross (MC).[4] He also served in North Africa, Sicily and Italy during the war. He was twice injured by a land mine. The one at Bou Arada in Tunisia placed him in bed for four months before he joined General Dwight D. Eisenhower's headquarters, where the Allied invasion of Sicily was being planned.[4] He won a Bar to his MC in 1944 at the Battle of Monte Cassino in recognition of "gallant and distinguished services",[4][5] and by the end of the war Jackson was in post as an acting major but was only formally promoted captain in August 1945,[6] having been promoted to lieutenant in 1940.[7] He was also mentioned in despatches in 1945 for his services in Italy.[8]
Jackson retired from his post in Gibraltar in 1982 (having had his tenure extended by a year) and returned to being historian at the Cabinet Office until 1987.[4] He had held five honorary military appointments: as ADC General to the Queen (1974–1979),[4] Colonel Commandant the Royal Engineers (1971–1981), Colonel the Gurkha Engineers (1971–1976), Colonel Commandant Royal Army Ordnance Corps (1973–1976)[2][24] and Colonel of the Territorial Army Volunteer Reserve Engineer and Railway Staff Corps.[25]