Enigma was not unbreakable. It looked unbreakable on first inspection — a typewriter-sized machine with 159 quintillion possible daily settings — but it had two structural flaws and one operational one, and a small team of Polish, then British, mathematicians turned those into a complete break by 1940. The Bombe is the machine that operationalised the British attack.
The Polish bomba 1938 · Poznań
Marian Rejewski, Jerzy Różycki, and Henryk Zygalski of the Polish Cipher Bureau spent the 1930s reading German Enigma traffic by exploiting a procedural weakness: every operator transmitted the message key twice at the start of each message. Rejewski's bomba kryptologiczna (literally "cryptologic bomb") of November 1938 mechanised the attack — six Enigmas linked together searching for consistent rotor positions. When the Germans changed procedure in September 1939, the Polish attack collapsed. Five weeks before the invasion of Poland, the Polish team handed everything they knew — including the rotor wirings — to British and French intelligence.
Alan Turing 1939 · Hut 8, Bletchley Park
Turing arrived at Bletchley on 4 September 1939 — the day after Britain declared war. The Polish bomba was already obsolete; the new German procedure had eliminated the doubled message key. Turing's insight was to attack the scrambler itself using cribs: probable plaintexts likely to appear in known positions. The British "Bombe" would test all 17,576 rotor starting positions, looking for one that consistently mapped the guessed plaintext to the observed ciphertext. The exploitable flaw: Enigma never encrypts a letter to itself.
"No one appreciates the half of what I do, half I get wrong anyway, and the others have heard it before."
— Turing, in a letter from Bletchley, 1942
Gordon Welchman's diagonal board 1940 · Hut 6
Turing's design worked but generated too many false stops to be practical. Welchman, the head of Hut 6, added a wiring scheme called the diagonal board in spring 1940 that propagated constraints across the entire crib graph simultaneously — exploiting Enigma's reciprocity (if A maps to B at some position, B maps to A at the same position). With the diagonal board, a long crib produced few enough stops to verify by hand. The British Tabulating Machine Company in Letchworth manufactured roughly 200 Bombes by 1945.
Operations 1940–1945 · Bletchley + outstations
Each Bombe stood 6 feet 6 inches tall and weighed about a ton. Operators — most of them young women drawn from the WRNS and ATS — set up the menu wiring, loaded the drums to the day's rotor configuration, and started the motor. A typical run took about 15 minutes. When the drums stopped, they wrote down the indicator letters and passed the candidate setting to Hut 8 (or Hut 6, for Luftwaffe traffic) where it was tested on a captured Enigma replica called a Type-X. If the decrypt was German, the day was broken.
By mid-1942 Bletchley was reading thousands of intercepts per day. The Battle of the Atlantic in 1943 turned partly on Bombe output. By 1944 the Bombes were running so much traffic that the outputs — codenamed Ultra — were considered the most valuable intelligence source the Allies had.
The afterlife 1945 onwards
Churchill ordered the Bombes destroyed at the end of the war. The official secrecy held for thirty years; it was only with F. W. Winterbotham's The Ultra Secret in 1974 that the British public learned what had happened at Bletchley. Most of the operators never spoke about their work. Turing was convicted of "gross indecency" in 1952 and chemically castrated; he died in 1954, almost certainly by suicide. The British government pardoned him in 2013.
Bletchley → Colossus → Manchester
The Bombe was electromechanical. By 1943 a German cipher even harder than Enigma — the Lorenz SZ machine, used by Hitler's high command — was being attacked at Bletchley by a different machine entirely: Colossus, designed by Tommy Flowers, the first electronic programmable digital computer. Ten Colossi ran at Bletchley by 1945. After the war, Max Newman and Alan Turing carried that work to Manchester, where the first stored-program computer ran a program in June 1948. The road from Antikythera to your laptop runs through this building.
Enigma's daily settings gave 159 quintillion combinations. The Bombe brute-forced 17,576 — what was left after Welchman's diagonal board reduced the search space — in fifteen minutes. The difference between 159 quintillion and "fifteen minutes" was three people in Poznań, one mathematician, and a few hundred WRNS operators.
Sources: Welchman, The Hut Six Story (1982); Sebag-Montefiore, Enigma: The Battle for the Code (2000); Copeland (ed.), Codebreakers (2010); Turing's Prof's Book (declassified 2012); plus Rejewski's published memoirs. Full list at docs/sources.md.