Malcolm Gladwell in The New Yorker:
On April 30, 1943, a fisherman came across a badly decomposed corpse floating in the water off the coast of Huelva, in southwestern Spain. The body was of an adult male dressed in a trenchcoat, a uniform, and boots, with a black attaché case chained to his waist. His wallet identified him as Major William Martin, of the Royal Marines. The Spanish authorities called in the local British vice-consul, Francis Haselden, and in his presence opened the attaché case, revealing an official-looking military envelope. The Spaniards offered the case and its contents to Haselden. But Haselden declined, requesting that the handover go through formal channels—an odd decision, in retrospect, since, in the days that followed, British authorities in London sent a series of increasingly frantic messages to Spain asking the whereabouts of Major Martin’s briefcase.
It did not take long for word of the downed officer to make its way to German intelligence agents in the region. Spain was a neutral country, but much of its military was pro-German, and the Nazis found an officer in the Spanish general staff who was willing to help. A thin metal rod was inserted into the envelope; the documents were then wound around it and slid out through a gap, without disturbing the envelope’s seals. What the officer discovered was astounding. Major Martin was a courier, carrying a personal letter from Lieutenant General Archibald Nye, the vice-chief of the Imperial General Staff, in London, to General Harold Alexander, the senior British officer under Eisenhower in Tunisia. Nye’s letter spelled out what Allied intentions were in southern Europe. American and British forces planned to cross the Mediterranean from their positions in North Africa, and launch an attack on German-held Greece and Sardinia. Hitler transferred a Panzer division from France to the Peloponnese, in Greece, and the German military command sent an urgent message to the head of its forces in the region: “The measures to be taken in Sardinia and the Peloponnese have priority over any others.”
The Germans did not realize—until it was too late—that “William Martin” was a fiction. The man they took to be a high-level courier was a mentally ill vagrant who had eaten rat poison; his body had been liberated from a London morgue and dressed up in officer’s clothing. The letter was a fake, and the frantic messages between London and Madrid a carefully choreographed act. When a hundred and sixty thousand Allied troops invaded Sicily on July 10, 1943, it became clear that the Germans had fallen victim to one of the most remarkable deceptions in modern military history.
More here.