“Are spies really of any value?” investigated (mostly) in the context of Operation Mincemeat, a deception to make Germany think an invasion in the Mediterranean would come through Greece, instead of Sicily.
A body that washes up onshore is either the real thing or a plant. The story told by the ambassador’s valet is either true or too good to be true. Mincemeat seems extraordinary proof of the cleverness of the British Secret Intelligence Service, until you remember that just a few years later the Secret Intelligence Service was staggered by the discovery that one of its most senior officials, Kim Philby, had been a Soviet spy for years. The deceivers ended up as the deceived.
But, if you cannot know what is true and what is not, how on earth do you run a spy agency? In the nineteen-sixties, Angleton turned the C.I.A. upside down in search of K.G.B. moles that he was sure were there. As a result of his mole hunt, the agency was paralyzed at the height of the Cold War. American intelligence officers who were entirely innocent were subjected to unfair accusations and scrutiny. By the end, Angleton himself came under suspicion of being a Soviet mole, on the ground that the damage he inflicted on the C.I.A. in the pursuit of his imagined Soviet moles was the sort of damage that a real mole would have sought to inflict on the C.I.A. in the pursuit of Soviet interests.