His older brother Ewen Montagu, a barrister in civilian life, became a Naval Intelligence Officer RNVR in MI6, during the Second World War, privy to the secrets of top-secret Ultra and the mastermind of the successful deception that launched the invasion of Sicily, Operation Mincemeat. He later wrote a best-selling account of that adventure, "The Man Who Never Was". Ivor Montagu himself turned out to be working, albeit briefly, for the other side. A 25 July 1940 cable from Simon Davidovitch Kremer, Secretary to the Soviet Military Attaché in London, identified him as the somewhat reluctant new recruit who was supposed to create an "X Group" of like-minded friends. By the following year, Hitler had invaded Russia and the Soviet Union became an ally of Britain's, so that by June, 1941, both brothers were technically working for the same side.