He joined the Territorial Army as a gunner in July 1939, and was commissioned in December the same year. After serving in the Norwegian Campaign in 1940, he joined the Prisoner of War Department at the War Office. He attended the Yalta Conference in 1944. The following year, he was promoted colonel and was appointed continental secretary of the British War Crimes Executive. He then served as leading junior counsel of the British prosecution team at the Nuremberg Trials in 1945–46. He was appointed OBE in 1946.