Operation Mincemeat was carried out in 1943. The Allied forces had just forced German and Italian forces out of northern Africa, and were beginning plans to invade Southern Europe by striking at Sicily and Italy.
Of course, the Axis were anticipating such an assault and were readying their defenses. To help conceal Allied intentions of definitively invading Sicily, the British engaged in a high stakes deception game with the Germans to determine where exactly in Europe the invasion would occur. The idea was to divide German forces across multiple defensive fronts, rather than concentrating their forces in just one area. Such a strategy would increase the odds of Allied victory.
The Allied plan was called Operation Mincemeat, where they decided to allow the Germans to discover top secret documents…which were in reality fake.
The British used the corpse of a homeless man named Glyndwr Michael, and turned him into Major William Martin. A submarine crew pushed the Boyd out into the water off the coast of Spain, and he was handcuffed to a briefcase with the fake top secret documents, among other items.
According to these fake documents, the Allied invasion was to occur in Greece. Just as the British had hoped, the Spanish authorities discovered the body and sent the documents to the German high command. Even though Spain was not formally involved in the war, they were sympathetic to the Axis cause.
The German High Command and Hitler both believed the documents, and concentrated heavy defenses in Greece and the Balkans as a result. This took away forces they could have used to defend Sicily, making it significantly weaker when the Allies launched their successful invasion in the summer of that year.