After retirement from the Army, he continued to be active in several areas. From 1968 to 1975, he was Principal of King's College, London. In 1978, he wrote a novel, ', which was a fictionalized scenario of the Third World War based on a Red Army invasion of West Germany in 1985. It was followed in 1982 by ', which elaborated on the original, including more detail from a Soviet perspective. An American author, Max Brooks, has cited Hackett's work as one source of inspiration for his novel, "World War Z".