Operation Mincemeat was a successful WW2 British deception operation to disguise the 1943 Allied invasion of Sicily. British intelligence obtained an unwanted body and took it to Hackney Mortuary where it was dressed as an officer of the Royal Marines and given personal items identifying him as the fictitious Captain (Acting Major) William Martin. Also placed on the body was correspondence between two British generals which suggested that the Allies planned to invade Greece and Sardinia, while pretending to invade Sicily.
The body was taken to Scotland and then by submarine close to the coast of southern Spain and released into the sea. It was picked up by a Spanish fisherman who took it to the authorities and the documents were shared with the Germans. Result: German reinforcements were shifted to Greece and Sardinia, leaving Sicily relatively unprotected, meaning there were fewer casualties when the Allies landed in July 1943.
The pathologist Sir Bernard Spilsbury advised on what state the body should be in to be convincing as the victim of an air crash and/or drowning and having spent some time in the sea.
This image from the National WW2 museum in New Orleans shows some of the ‘pocket litter’ that was "created for Martin – various bits and bobs that would go inside the man’s jacket and wallet that would add flavour to his story and help convince the Spanish and the Germans that he was a real man."
The Operation was, at least in part, based on a idea of Ian Fleming's.