Poland handed Enigma secrets to allies before invasion

Through the 1930s, Polish military mathematicians led by Marian Rejewski had reconstructed the German Enigma machine's wiring and built working decryption methods, including the electromechanical 'bomba' device, giving Poland a working method for reading Enigma traffic years before British or French cryptanalysts had made comparable progress.

The conventional instinct for an intelligence service holding a unique advantage is to guard it as long as possible, sharing nothing until forced to by circumstance or alliance obligation, since revealing methods risks losing whatever edge remains. Poland instead had to weigh that instinct against a starker fact: whatever advantage it kept secret would very likely disappear entirely the moment the Wehrmacht crossed the border.

Handed over reconstructed Enigma machines, methods, and bomba design to allies before invasion, transferring an edge that would evaporate with the country's fall.

Invasion was imminent, so the edge would be lost if not shared.

The advantage was informational, so it could be copied without loss.

Allies had the incentive and capability to use it immediately.

When you can't exploit an advantage before a deadline, transfer it to someone who can, rather than lose it.

British codebreakers, still behind Poland's progress, left the July 1939 meeting with methods historians credit with saving crucial years. GCHQ's own official history describes the Pyry meeting as transforming British cryptanalysis of Enigma virtually overnight, and Alan Turing's subsequent work with Polish mathematician Rejewski in Paris in January 1940 built directly on what Poland handed over rather than starting British efforts from first principles.