From the reign of Elizabeth I to the Cold War, Great Britain has effectively used codebreakers to turn the tide in its military engagements. This history explores the use of ciphers during the Napoleonic wars, the role of the Royal Mail's Secret Office, and the activities of the Admiralty's "Room 40" leading to the creation of the Government's Code and Cypher School. Front and center here is Alan Turing's Bletchley Park codebreaking operation, which broke Germany's Enigma code, and related efforts to decipher Japan's Purple code system.