What vital role in World War 2 was played by Major William Martin, Royal Marines?

In 1943 it became essential to deceive the Germans in thinking that the next major action after North Africa would be in Greece rather than in Sicily. So British Intelligence staged an aircraft accident off the coast of Spain in which a middle-ranking officer "died" while carrying secret orders. But the officer never existed. The genuine corpse picked up by the Spanish patrol had been given a very carefully constructed false identity as well as false papers in the attached briefcase. The body had been selected from a London morgue. British Intelligence knew that the Spanish authorities would pass on the supposedly authentic information to the Germans. The deception worked.

With grim irony the authorities called this "Operation Mincemeat"; it later became the subject of a book and movie "The Man Who Never Was".