Reilly's most skeptical biographer, Andrew Cook, asserts that Reilly's SIS-specific career has been greatly embellished as he wasn't accepted as an agent until 15 March 1918. He was then discharged in 1921 because of his tendency to be a rogue operative. Nevertheless, Cook concedes that Reilly previously had been a renowned operative for Scotland Yard's Special Branch and the Secret Service Bureau which were the early forerunners of the British intelligence community. Historian Christopher Andrew, a professor at University of Cambridge with a focus on the history of the intelligence services, described Reilly's secret service career overall as "remarkable, though largely ineffective".