In A Nutshell
Operation Cornflakes was a mission undertaken by the OSS as a means to spread propaganda throughout Germany. The envelopes containing the materials had fake German stamps affixed to them and they were delivered to their destinations by the German postal service, which had been duped into doing so. However, it is unlikely the operation did anything other than annoy German mail carriers.
The Whole Bushel
One of a number of psychological operations (PSYOP, for short) devised by the Office of Strategic Services, the forerunner of the American CIA, Operation Cornflakes was focused on trying to sway the hearts and minds of the German civilian population through propaganda. The idea was that each morning, when a German would eat his or her cornflakes, a piece of pro-Allied writing would be next to the bowl.
In order to get the pamphlets and other materials to their intended destinations, the OSS came up with an ingenious plan: They would send one plane to bomb a German mail train and another one to drop bags full of stuffed envelopes with forged stamps affixed to them. (A version known as the “death’s head” stamp was also created, which depicted Hitler’s head, with his bones partially exposed.) One of the only recorded successful dispersal missions took place in the summer of 1945 when nearly 4,000 envelopes were dropped near Linz and subsequently delivered.
As far as the stamps themselves go, two CIA agents gave reports which indicated the British forgeries were far superior to the French and Americans, since they were manufactured in regular postage stamp factories. However, wartime problems, such as supply shortages and coloring issues, led to a number of stamps which were unusable, since they could be easily recognized as fakes.
Operation Cornflakes most likely had little to no effect, since the war was nearly over and morale seems to be lowered by more immediately recognizable forces, such as repeated bombings or food shortages.
Show Me The Proof
CIA: Postal Forgeries
Office of Strategic Services 1942-45: The World War II Origins of the CIA, Eugene Liptak
Poison Cornflakes for Breakfast