"Colossus was the world's first electronic programmable computer, located at Bletchley Park in Buckinghamshire. Bletchley Park was the British forces' intelligence centre during WWII, and is where cryptographers deciphered top-secret military communiques between Hitler and his armed forces. The communiques were encrypted in the Lorenz code which the Germans considered unbreakable, but the codebreakers at Bletchley cracked the code with the help of Colossus, and so aided the Allies' victory."
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For a more detailed discussion of WWII code breakers, see The Code Book, by Simon Singh.