The second factor was the apparent quality and authenticity of the documents themselves. The Nye letter was written by a real general, in his actual voice, on appropriate stationery, with the kind of language that a real senior British military officer would use in private correspondence. German analysts who examined the document would find no obvious seams or inconsistencies. The personal effects that accompanied Martin were similarly authentic-seeming — not a polished set of props, but the ordinary detritus of a real person’s life.
The third factor was the route through which the documents arrived. They did not arrive through an agent whose reliability might be questioned or through a channel with a history of producing suspect material. They arrived as apparently accidental documents on the body of a dead officer recovered from the sea. Accidental intelligence — material that appears to have fallen into enemy hands by chance rather than through an established intelligence channel — carries a special kind of credibility, because it is harder to suspect of being planted.
The fourth factor was the Ultra feedback mechanism. The British were monitoring German intelligence assessments in real time and could calibrate their response to what they observed. When German intelligence appeared to be accepting the Mincemeat material, the British could reinforce the deception through other channels. When German assessments showed uncertainty, the British could consider how to address it.
The fifth factor was the broader deception system within which Mincemeat operated. The documents’ claims were being reinforced, through entirely separate channels, by double agents and by other components of Operation Barclay. German intelligence was not receiving a single piece of evidence pointing to Greece and Sardinia. It was receiving a coherent picture from multiple sources, all of which happened to be shaped by British intelligence.