They think it was in World War II. They don’t know if the message, written in blue ink and sent by homing pigeon, was a desperate combat request or a routine training evolution. Thanks to the original sender having put two Pigeon IDs on the message, they aren’t even sure what pigeon it was.
They just know the carrier pigeon never got through.
They know that because David Martin of Surrey, England found the remains of the bird in his chimney. Along with the dessicated bird, there was its message container — and a legible message. But what did it say? The text of the message was — and is — enciphered.
Unfortunately, attempts to read the enciphered message that the doomed bird carried have failed. Bletchley Park, where German codes were solved during the war and where a cryptologic museum is today, took a shot at it, and then forwarded it to the Government Communications Headquarters (GCHQ) in Cheltenham. GCHQ is one of the most advanced code-breaking organizations in the world, but they couldn’t crack the code.
Some time after the war, the original code- or cipher-books, no longer used with pigeon messaging obsolete, were destroyed; they still posed a significant security risk as long as any message encrypted in the cipher might still be in hostile hands.
There is much more information in this Wall Street Journal story by Cassell Bryan-Low. (It was news to us that the British used carrier pigeons in the Second World War, as it’s a technology we always associated with the First). Bryan-Low’s article also contains a link to a downloadable .pdf of the mystery message if you feel like putting your cryptological chops to the test. The Bletchley Park museum is waiting to hear from you.