The trick was that a corpse with no identification and a bunch of maps of Greece would seem like a fairly obvious trap to German intelligence and quickly dismissed as misinformation. For Mincemeat to work, it had to seem real. It had to seem like a military courier with important documents had crashed into the ocean and drowned. Montagu and the other Naval Intelligence officers knew that, so they spared no effort in making Captain William Martin, the identity they chose for their fake officer, seemed completely believable.
To begin with, they picked the name William Martin because there were several men in the Royal Marines with the same name and rank, which they hoped would make it difficult for the Germans to sort out whether or not all the Will Martins were alive at the time the body was found. Then, Montagu carefully constructed a fake life for their fake officer. They included a photo in the body’s pockets of a fake fiancée named Pam along with some love letters, a receipt from a local jeweler for a diamond engagement ring, and even a statement from a bank saying that Martin’s account was overdrawn.
And along with the fake love story Montagu created, he even went so far as to pay attention to the most mundane details. To make it seem like Martin was a real person, Montagu stuffed the body’s pockets with the sort of things that everyone has in their pockets but no one ever thinks about like a set of keys, a pencil, cigarettes, and even a receipt from a new shirt. Finally, Montagu created a fake identity document using a photo of an MI6 agent who bore a resemblance to Michael, who then spent weeks carefully rubbing the document against his pants to give it a worn-down appearance.
With the identity of William Martin established, MI6 then needed to find a way to trick the Germans into thinking that Martin had some inside information about Allied invasion plans. Again, this sort of espionage required a great deal of subtlety. So, rather than have Martin carry any official military documents, the planners of Mincemeat decided that he should be carrying an informal letter between General Archibald Nye and General Harold Alexander, two British commanders involved in planning the invasion.
The letter was purposely vague, making only off-hand remarks about German moves in Greece that threatened “the assault” and British attempts to reinforce their troops in the region to counter them. The value of this sort of letter is that it wasn’t direct enough to make the planned invasion look obvious. Instead, it was the kind of letter two generals and friends might send each other with just enough information about a planned assault to make the Germans feel confident that they had scored an intelligence coup. And with the perfect letter in hand, the only step left was to figure out how to get it to the Germans.