On April 30, 1943, the body of Glyndwr Michael, disguised as Major William Martin of the Royal Marines, was dropped into the sea off Huelva on the Spanish coast. In his briefcase were letters, meticulously faked by British intelligence officers to give the impression that the Allies intended to attack Greece, and not Sicily.
So begins the remarkable story of “Operation Mincemeat” – an amazingly complicated and surprisingly successful operation by MI5. After finding a suitable”volunteer” from the among the vagrants’ corpses in a London morgue, operatives created a detailed back story and a convincing set of personal effects and letters. A submarine deposited “Major Martin” a mile from shore with his attache case. Many twists and turns followed, even after the Germans took the bait. Here’s a thorough-seeming Wikipedia page on the operation.