MI5’s director of counterespionage from 1940 to 1945, Liddell had been decorated with the Military Cross during World War I and later had joined Scotland Yard’s Special Branch as a civilian analyst concentrating on Soviet espionage. In 1931 Special Branch’s staff was amalgamated with the Security Service, and Liddell was appointed an assistant to Brigadier Jasper Harker, the director of B Division, succeeding him in June 1940.
Socially well connected, having married into the Barings banking family, Liddell oversaw MI5’s dramatic growth during the first 18 months of the war as the threat from Fifth Columnists, enemy aliens living in Britain, the fear of a Nazi invasion, and an Abwehr espionage offensive all posed a threat to the country. Liddell’s actions and opinion were documented by him in 12 volumes of a daily journal that he dictated from August 1939 to June 1945. Code-named WALLFLOWER, these sensitive documents were retained in the safe of the director-general until they were declassified and then published in 2005.
After the war, Liddell was appointed deputy director-general of MI5, but in 1953 transferred to the Atomic Energy Authority as a security adviser. The end of his career had been marred by a close wartime friendship with Anthony Blunt, who had served as his personal assistant, and by his friendships with the Cambridge Five spies Guy Burgess and Kim Philby. He died in 1958, seven years after the defections of Burgess and Maclean, aware only that Blunt had fallen under suspicion but not that he would confess to having spied for the Soviets throughout World War II.