Montagu played a leading role in the 1943 scheme to deceive the Germans about the planned Allied invasion of Sicily.
The scheme entailed releasing a dead body just off the coast of Spain, where strong currents caused it to drift ashore in an area where a skilled German secret agent was known to operate.
The corpse was to appear to be the victim of an airplane crash, the non-existent Royal Marine Captain (Acting Major) William Martin, who had letters in a briefcase that hinted at a forthcoming Allied invasion of Greece and Sardinia, rather than the obvious target of Sicily.
In 1950 Duff Cooper, a former cabinet minister who had been briefed on the operation in March 1943, published the spy novel Operation Heartbreak, which contained the plot device of a corpse – with papers naming him as William Martin – being floated off the coast of Spain with false documents to deceive the Germans.
He was also careful to obscure "the idea of an organised programme of strategic deception ... with Mincemeat being presented as a 'wild' one-off caper".