The Man Who Never Was (Ronald Neame, 1956) I first read the story of the off-beat plan to distract the Nazis from British invasion plans in grade-school with a scholastic printing of Ewen Montagu's fascinating book. I knew there'd been a short-lived TV series with Robert Lansing in the 60's—more live-spy than the actual event—but this had nothing to do with that. This one was a true story. It started a fascination with the morally ambiguous world of spies (in WWII and beyond) that still fills my trench-coat to this day.
But this story was true; the name of the corpse washed ashore was changed to protect the Allies on the invasion route to Sicily.
On April 30, 1943, the body of Major William Martin of the Royal Marines was washed ashore on the beach of Huelva, Spain. Chained to the loop of his trench-coat was a briefcase containing personal documents (so they wouldn't be transferred through official channels) that hinted that the invasion, code-named Operation Husky, would land at Greece and Sardinia, and that deceptive intelligence efforts would be made to convince the Germans that the invasion would take place attacking Sicily.
It was all a hoax. There was no Major Martin (although his obituary did appear in The London Times), and the body was a plant by British intelligence forces to sway the Germans from the actual Husky invasion of Sicily, a strategic "must" for the invasion of first Italy, and then the Eastern pincer move on Germany. (Winston Churchill once remarked that "Everyone but a bloody fool would know that it's Sicily"). The plan, dubbed "Operation Mincemeat"—in the hope that the Germans would "swallow" it—was concocted by Flight Lt. Charles Cholmondeley* and Lt. Cmdr. Montagu. A corpse (that of a 34 year old Welshman Glyndwr Michael, who had no immediate family) was obtained, an elaborate back-history created, documents forged (including love letters and family correspondence), and dumped by submarine off the Spanish coast.
The Germans bought it...to the point where they still anticipated the attack two weeks after the Allies landed in Sicily. The film of the book by Montagu (played in the film by Clifton Webb) takes a few liberties—it romanticizes some of the incidents (for instance the authorship of the love letters) and creates a follow-up operation that ensures the information is transferred by a German agent (Stephen Boyd, in his film debut) without embellishment to the enemy, but it's an interesting dramatization ("It's the most outrageous, disgusting, preposterous, not to say barbaric idea I've ever heard, but work out full details and get back to me in the morning!") about one of the wierder operations in a war full of them.
* Cholmondeley got the idea from a multi-schemed memo by one intelligence officer named Ian Fleming (who was inspired by a novel by Basil Thomsen), who in 1953 would create his own intelligence agent, James Bond.