The Man Who Never Was (1956)

Alan Bellows via Damn Interesting:

The idea to plant false military documents on a dead man and let them fall into the hands of the Germans was conceived by Lt. Cmdr. Ewen Montagu at British naval intelligence. His was a variation of an earlier idea proposed by Flight Lt. Charles Cholmondeley of the counter-intelligence service MI5. Cholmondeley had suggested that a wireless radio could be placed on a dead soldier whose parachute was rigged to appear to have failed, which would provide the Allies with a channel to provide disinformation to the enemy. But his plan was deemed impractical, so Montagu's death-at-sea ruse was implemented instead, and dubbed Operation Mincemeat.

Montagu's team quietly procured the body of a 34-year-old man who had recently died with pneumonia, whose lungs already contained fluid as a drowned man's would. The family of the deceased granted permission to use the body for this mission on the condition that the man's identity never be revealed. As the body waited in cold storage, the fictional life of Major William Martin was fabricated in great detail by the Twenty Committee (often referred to by the roman numeral XX or "double-cross"). The corpse was given identification, keys, personal letters, and other possessions. In order to explain why the man would be found chained to his briefcase, Montagu's team planted evidence suggesting that Major Martin was an absent-minded but responsible chap, including overdue bills and a replacement ID card. Such a man might chain himself to a briefcase full of sensitive documents in order to prevent its loss during the flight.

On 28 April 1943, Major Martin was placed aboard the submarine HMS Seraph in a special steel canister packed with dry ice. The crew set off for the coast of Spain, where it was likely that a citizen of the Axis-aligned country would locate the body and report it to authorities. After two days at sea, the submarine surfaced about a mile off the coast of Spain at 4:30 in the morning. Believing that the heavy canister contained top secret meteorological equipment, members of the crew carried it on deck, after which point everyone aside from the officers was ordered below deck. There in the dark, Lt. Norman L.A. (Bill) Jewell, the commander of Seraph, explained the mission and swore the men to secrecy. Major Martin's body was then removed from the canister onto the deck, where he was fitted with his life jacket and chained to his briefcase. The men read the 39th Psalm and committed the body to the sea, where the tide gradually drew it ashore.

Once the body was discovered, Britain's requests for the return of the briefcase helped complete the illusion that there was sensitive information contained therein. To further the hoax, Montagu arranged to have Major Martin's name included on the next British casualty list in The Times. When the documents were finally returned to the British two weeks later, microscopic examination revealed that the Germans had indeed opened and resealed the letters. Additionally, German transmissions decrypted by Ultra indicated that the Nazis were moving forces to defend Sardinia, Corsica, and Greece. This news prompted a brief cable to Winston Churchill to inform him of the success: "Mincemeat Swallowed Whole."