Enigma machine 1943; Wikimedia Commons

The operator of the German Enigma machine  (invented by a Dutchman in 1919 and adapted by Germany for military use) typed a message that was scrambled by three to five rotors that displayed different letters of the alphabet. With exact knowledge of the transmitter’s rotor settings, the receiver could reconstitute the original text. Throughout the war, the Enigma machine became more complicated, as Germans introduced various quirks and new electronic circuits.

Germany considered the continually changing code produced by the typewriter-like Enigma machine to be virtually unbreakable. But, unbeknownst to Germany, British cryptologists at Bletchley Park first broke the code during the German invasion of Poland in 1939.  In July 1941 the Enigma code used in ground-to-air operations on the Eastern front was also deciphered and the messages were shared with Russia.

In 1941, U-boat  attacks on ship convoys in the North Atlantic posed the greatest threat to the Allied war effort. On December 8, 1941 the more complex Enigma code used by the German Abwehr (secret service) was successfully broken allowing the interception of messages pertaining to the control and  location of submarines in the Atlantic, information about bombing raids, troop movements, and the location of military supply ships.

In February 1942 Germany introduced a new fourth wheel into their Naval Enigma machine. During the year it took Bletchley Park to break the new code, Allied losses in the Atlantic sharply increased. It was not until until August  1943 that German Naval Enigma was reliably decoded again.