From Wikipedia: Operation Mincemeat was a very successful British deception plan during World War II. Mincemeat convinced the German high command that the Allies planned to invade Greece and Sardinia in 1943 instead of Sicily, the actual objective. This was accomplished by persuading the Germans that they had, by accident, intercepted "top secret" documents giving details of Allied war plans. The documents were attached to a corpse deliberately left to wash up on a beach in Spain. The story was revealed in the 1953 book The Man Who Never Was.
Brief Historical Overview: In 1942, Operation Torch was imminent, and victory in the North African Campaign was expected. Allied planners considered the next step in the war. They decided to continue attacks in the Mediterranean theatre. Control of Sicily would open the Mediterranean to Allied shipping and allow invasion of continental Europe, making Sicily an obvious strategic objective. German planners saw this too, of course. (Winston Churchill commented "Everyone but a bloody fool would know that it's Sicily.") Furthermore, there would be a massive Allied buildup for the invasion (code-named Operation Husky) that would surely be detected. The Germans would know that some large attack was coming. But if the Allies could deceive the Germans about where that attack was going, the Germans might disperse or divert some significant part of their forces, which would help the invasion succeed.
Several months before, Flight Lt. Charles Cholmondeley of Section B1(a) of MI5, suggested dropping a dead man attached to a badly-opened parachute in France with a radio set for the Germans to find. The idea was for the Germans to think that the Allies did not know the set was captured, and pretend to be Allied agents operating it, thus allowing the Allies to feed them misinformation. This was dismissed as unworkable; however the idea was taken up later by the Twenty Committee, the small inter-service, inter-departmental intelligence team in charge of double agents. Cholmondeley was on the Twenty Committee, as was Lt. Cmdr. Ewen Montagu, a Royal Navy intelligence officer.Montagu and Cholmondeley developed Cholmondeley's idea into a workable plan, using documents instead of a radio. The Committee thought of planting the documents on a body with a defective parachute. However, the Germans knew that it was Allied policy never to send sensitive documents over enemy territory, so they decided to make the man a victim of a plane crash at sea. That would explain how the man would be several days dead and how he could be carrying secret documents. The body would be floated ashore in Spain, where the nominally neutral government was known to cooperate with the Abwehr (German intelligence). The British were sure the Spanish authorities would search the body and allow German agents to examine anything found. Montagu gave the operation the code name of Mincemeat, just restored to the list of available names after its use for another successful mission.