Battles & Wars › Between the wars › Bletchley Park and Ultra codebreaking
Enigma machine
patented 1918; German military from 1926; broken by Poland 1932· Invention or discovery· Technology, codes & intelligence
Well documentedSolid records, physical evidence or scientific consensus.
A German electro-mechanical cipher machine whose rotating wheels changed the code with every key press; Polish, then British, codebreakers broke it.
See it on the Battles & Wars map →What happened
Engineer Arthur Scherbius patented Enigma in 1918 and sold it to businesses before the German navy (1926) and army (1928) adopted it. Pressing a key sent current through three stepping rotors and a plugboard, lighting a different letter each time; the army version had about 159 quintillion possible settings. Germany thought it unbreakable. But in December 1932 Polish mathematician Marian Rejewski worked out its wiring, helped by documents a German spy had sold to France.
Why it matters
Breaking Enigma gave the Allies 'Ultra,' a steady window into German military orders, above all in the U-boat war, and drove Bletchley Park's code-breaking machines, an early step toward computers.
How we know
Surviving machines, Scherbius's patents, Polish Cipher Bureau records, Rejewski's later accounts, Gustave Bertrand's 1973 book, and Bletchley Park files declassified from the 1970s.
Still debated
How much Ultra shortened the war is debated; official historian Harry Hinsley's estimate of at least two years is often quoted but disputed.
Connections (5)
▸Ancient ciphers (Spartan scytale, Caesar cipher) evolved over two millennia into the Enigma machineMostly agreed
The connection
Caesar shifted each letter by a fixed amount, a method the Arab scholar al-Kindi showed in the 800s could be cracked by counting letter frequencies. Renaissance ciphers fought back by changing the substitution letter by letter. Enigma automated that idea: its rotors stepped with every key press, giving a new scrambled alphabet for each letter.
When and how
Rome, 50s BC; Baghdad, about 850 (al-Kindi); Rome, 1467 (Leon Battista Alberti's cipher disk); Berlin, 1918 (Scherbius's patent).
How sure are we?
Suetonius describes Caesar's cipher; al-Kindi's and Alberti's treatises survive; Scherbius's patent shows the rotor method.
Debates and open questions
Whether the Spartan scytale was really a cipher device or just a message staff is debated, so the oldest link in the chain is uncertain.
▸Enigma machine was broken on an industrial scale at Bletchley Park and Ultra codebreakingWell documented
The connection
Enigma scrambled each letter through rotating wheels and a plugboard, with settings changed daily, giving an astronomical number of possible keys. Polish codebreakers led by Marian Rejewski broke it first and shared their methods with Britain and France in July 1939. Bletchley turned this into a production line: intercept stations, Bombes and huts working around the clock.
When and how
Pyry, near Warsaw, July 1939; Bletchley Park, 1939–45; Rejewski, Jerzy Różycki and Henryk Zygalski, then Turing, Gordon Welchman and thousands of staff.
How we know
Declassified Bletchley histories, Polish accounts, and surviving Enigma machines and a rebuilt Bombe at Bletchley Park.
Debates and open questions
German operator errors, captured codebooks and predictable message formats mattered as much as machines; how much each contributed is hard to separate.
▸Enigma machine had already been broken by Poles before the German invasion of PolandWell documented
The connection
Poland's Cipher Bureau read German Enigma messages for much of the 1930s, until new German procedures in late 1938 swamped its methods. On July 25–26, 1939, five weeks before the invasion, the Poles handed French and British codebreakers their methods and two replica Enigmas. After Poland fell, its codebreakers escaped to France and kept working.
When and how
Pyry forest near Warsaw, July 25–26, 1939; Marian Rejewski, Jerzy Różycki and Henryk Zygalski; Gustave Bertrand of France; Alastair Denniston and Dilly Knox of Britain.
How we know
Rejewski's and Bertrand's accounts, British records of the meeting, and Bletchley Park histories crediting the Polish start.
Debates and open questions
How many months the Polish gift saved Bletchley is debated, but most historians call it crucial.
▸Alan Turing designed the Bombe to break the Enigma machineWell documented
The connection
Turing saw that likely words in a message, called cribs, such as standard weather reports, could be used to rule out wrong Enigma settings by logic. His Bombe, improved by Gordon Welchman's 'diagonal board', tested settings mechanically at high speed. The first, named Victory, ran in March 1940; over 200 were eventually built in Britain.
When and how
Bletchley Park, late 1939 – 1940; designed by Turing with Welchman and engineered by Harold 'Doc' Keen of the British Tabulating Machine Company in Letchworth.
How we know
Turing's wartime 'Treatise on the Enigma', declassified in the 1990s, and surviving Bombe designs; a working rebuilt Bombe runs at Bletchley Park.
Debates and open questions
How much the British Bombe borrowed from the Polish 'bomba' is discussed; it shared the name and goal but worked on a different principle.
▸Enigma machine encrypted the U-boat orders of the Battle of the AtlanticWell documented
The connection
U-boats reported positions and received orders from Dönitz's headquarters by radio, encrypted on naval Enigma, which was harder to break than army versions. Wolfpack tactics depended on this constant radio traffic. That made the U-boat fleet both effective and vulnerable: whenever Bletchley could read it, convoys could be routed away.
When and how
U-boat headquarters in France and Germany and boats at sea, 1939–45; the three-rotor naval Enigma and, from February 1942, the four-rotor M4.
How we know
German naval radio logs and captured machines and codebooks, notably from U-110 and U-559, confirm how the system worked.
Debates and open questions
Why German naval intelligence, which had its own suspicions, never concluded that Enigma was broken is still discussed.