I. THE WAITING PERIOD AND THE ASSESSMENT OF DECEPTION SUCCESS
The period between the deployment of the body on April 30, 1943, and the actual invasion of Sicily on July 9-10, 1943, was a period of intense uncertainty for the people who had run Operation Mincemeat.
The planners knew that the documents had been delivered. They did not know, initially, whether the documents had actually reached German intelligence. They did not know whether the documents had been believed. They did not know whether any changes in German defensive dispositions could be attributed to the deception rather than to other factors. And they could not ask anyone.
The intelligence assessment was conducted entirely through indirect means. The primary source was Ultra — the signals intelligence derived from breaking German military communications. Ultra intercepts allowed British analysts to monitor, in broad terms, German assessments of Allied intentions in the Mediterranean and to track changes in German military dispositions.
The process of reading Ultra material was itself highly classified and extremely sensitive. Only a small number of people in the relevant commands knew that British intelligence was reading German communications. The existence of Ultra was not publicly known until 1974, which is why Montagu’s 1953 account of Operation Mincemeat could not explain exactly how the British knew the deception had succeeded. He knew, because he was briefed on the relevant intelligence, but he could not say so without compromising one of the most important intelligence secrets of the war.
The first indicators came fairly quickly. Within days of the body’s recovery, German intelligence in Spain was reporting on the documents, and those reports were being intercepted. The specific content of the intercepted messages, as later declassified and analyzed, showed that German intelligence had taken the documents seriously and was assessing their implications.