The problem in 1943 was that Sicily was obvious. It was the natural stepping stone from North Africa to Italy, and Churchill remarked that anyone but a damned fool would know it was Sicily.
The solution came from a note in the Trout Memo, a 1939 list of deception ideas circulated under the name of the head of naval intelligence and probably written in part by his assistant Ian Fleming. Idea number twenty-eight suggested planting a corpse carrying misleading papers where the enemy would find it.
Ewen Montagu and Charles Cholmondeley of the Twenty Committee built it out.
They obtained the body of a man who had died in London — Glyndwr Michael, a homeless Welshman who had ingested rat poison — and gave him an identity. Acting Major William Martin of the Royal Marines. And then they gave the identity a life.
This is the part that matters. The body carried a photograph of a fiancée called Pam, two love letters from her, a bill from a jeweller for an engagement ring, an overdrawn notice from Lloyds Bank, ticket stubs from a London theatre dated shortly before he supposedly flew, a receipt for a new shirt, a letter from his father, an expired identity card with a replacement, and a set of keys. The theatre stubs were selected so that the dates would place him in London at a plausible moment.
The operational documents were the point of the exercise: a private letter from one general to another, discussing an Allied plan to attack Greece and Sardinia and noting that Sicily was to be the cover target.
He was released from a submarine off Huelva on 30 April 1943, in Spanish waters, where a pro-German intelligence apparatus operated and the material would be passed to the Abwehr.
Two mechanisms made it work.
Effort. Information a target believes it has obtained by its own effort, or by the enemy's mistake, is weighted far more heavily than the same information offered. Nothing was given to the Germans; they took it, and taking it was a small achievement they had earned.
And schema fit. The pocket litter is not corroboration in any evidentiary sense — a forged document and a forged theatre stub have identical evidentiary value, which is none. What the litter does is make the person coherent, and coherence is what people use as a proxy for truth when verification is impossible. The Germans were not evaluating documents. They were recognizing a man.
Hitler ordered reinforcements to Greece and Sardinia. When Sicily was invaded on 10 July, the initial German assessment was that it was the diversion.
Aftermath & Cross-Reference: Planted Evidence Defence