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Why was Colossus built during World War II?

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The Colossus computer at Bletchley Park, Buckinghamshire, England, c. 1943. — Colossus Funding for this code-breaking machine came from the Ultra project.

Colossus was built during World War II to serve as a high-speed, electronic tool for code breaking at Bletchley Park, the British government's wartime intelligence headquarters.12

Specifically, the machine was designed to decrypt high-level German military transmissions known as "Tunny." These messages, encrypted by the Lorenz SZ cipher machine, were used by Adolf Hitler and the German high command to communicate with field marshals and generals. While British code breakers had developed manual methods to decipher these transmissions, the volume of intercepted messages increased, and German security enhancements made manual decryption too slow to be effective.2

To address this, engineer Tommy Flowers developed Colossus to automate the fundamental stages of the Tunny code-breaking process at electronic speeds. By replacing earlier, unreliable electromechanical equipment like the "Heath Robinson," Colossus provided the Allies with critical intelligence regarding German battle orders, supplies, and personnel, which proved vital to the success of the Normandy D-Day landings and the subsequent advance across Europe.12