Paul Gannon’s book on Colossus reveals the previously closely-guarded story of how British codebreakers at Bletchley Park broke into the secret communications of Hitler and his high commanders. The breaking of the Enigma machine at Bletchley Park is well known. But there was another, more complex and much more secret German cipher machine, the Geheimschreiber (‘secret writer’) used by Hitler and his high command to communicate with armies in NW Europe, the Mediterranean and Russia. Colossus was invented to break this more sophisticated cipher machine and in doing so revealed the details of Hitler’s operational planning to Allied armies. Colossus was the world’s first large-scale electronic computing machine - a pre-runner of the electronic computer and made Bletchley Park the world’s first ‘electronic information factory’. Paul Gannon’s book is the first to place Bletchley Park in its full historical context.