Juan Pujol García was a Spaniard who hated fascism and decided, on his own initiative, to work against Germany. He approached the British three times and was turned down each time.
So he went to the Germans instead, offered to spy for them in Britain, was accepted, and then — without ever going to Britain — invented what he reported. He wrote from Lisbon, using a Blue Guide to England, a Portuguese book on the Royal Navy, a railway timetable and an English-French vocabulary of military terms. His reports contained errors that ought to have been obvious, including expenses claimed in a currency that did not exist and a claim that men in Glasgow would do anything for a litre of wine.
The Germans believed him. British intelligence, meanwhile, noticed unexplained German naval activity responding to a convoy that did not exist, and eventually worked out that someone was inventing an agent network. They found him, and in 1942 brought him to Britain.
Under Tomás Harris of MI5, Pujol became GARBO, and the network became a machine. Twenty-seven notional sub-agents, each with a personality, a location, a specialization and a plausible reason for having access. When one of them needed to fail to report something, he was given an illness; one, a Liverpool-based agent, was killed off and an obituary placed in a local paper, and the Germans paid a pension to his fictitious widow.
The strategy is the thing to study, and it is the same one every long confidence operation uses.
Truth was spent to buy credibility for one lie. GARBO's reports were largely accurate, and this was deliberate: the Twenty Committee released real information, sometimes genuinely damaging, timed so that it arrived too late to be operationally useful but early enough to be verified afterwards. Over two years the Abwehr built a track record, and Hovland's source-credibility research describes what a track record does: it comes to dominate assessment of any individual message, so that plausibility is judged against the source rather than against the world.
By 1944 his handlers were also sunk-cost committed. GARBO was their best asset, their institutional achievement, and their reputation.
On 9 June 1944, three days after the Normandy landings, GARBO sent the message the entire operation existed for: that Normandy was a diversion and the main assault would come at the Pas de Calais. The Fifteenth Army stayed where it was.
He was awarded the Iron Cross by Germany and made an MBE by Britain.
Aftermath & Cross-Reference: Source Vetting Protocol