During World War II, British intelligence officers managed to pull off one of the most successful wartime deceptions ever achieved: Operation Mincemeat.

In April 1943, a decomposing corpse was discovered floating off the coast of Huelva, in southern Spain. Personal documents identified him as Major William Martin of Britain’s Royal Marines, and he had a black attaché case chained to his wrist. When Nazi intelligence learned of the downed officer’s briefcase (as well as concerted efforts made by the British to retrieve the case), they did all they could to gain access.

Though Spain was officially neutral in the conflict, much of its military was pro-German, and the Nazis were able to find an officer in Madrid to help them. In addition to other personal effects and official-looking documents, they found a letter from military authorities in London to a senior British officer in Tunisia, indicating that Allied armies were preparing to cross the Mediterranean from their positions in North Africa and attack German-held Greece and Sardinia.

This intelligence coup for the Nazi spy network allowed Adolf Hitler to transfer German troops from France to Greece ahead of what was believed to be a massive enemy invasion.

The only problem? It was all a hoax.

The “drowned” man was actually a Welsh tramp whose body was obtained in a London morgue by British intelligence officers Charles Cholmondeley and Ewen Montagu, the brains behind Operation Mincemeat. After creating an elaborate fake identity and backstory for “William Martin,” Cholmondeley and Montagu got Charles Fraser-Smith (thought to be the model for Q in the James Bond novels, written by former British naval intelligence officer Ian Fleming) to design a special container to preserve the body during its time in the water. One of England’s leading racecar drivers transported the container to a Royal Navy submarine, which dropped it off the Spanish coast.

Once the Spanish recovered the body, British authorities began their frantic attempts to recover the case, counting on the fact that their efforts would convince the Nazis of the documents’ validity. As a result of the false intelligence carried by “William Martin,” the Nazis were caught unawares when 160,000 Allied troops invaded Sicily on July 10, 1943. In addition to saving thousands of Allied soldiers’ lives, Operation Mincemeat helped further Italian leader Benito Mussolini’s downfall and turn the tide of the war towards an Allied victory in Europe.