When the German cruiser Magdeburg ran aground off the Estonian coast in August 1914, little did the British Admiralty realise that what was onboard the ill-fated vessel would hand them a decisive advantage in the battle for North Sea supremacy and, ultimately, war-winning opportunities. On board the Magdeburg was a collection of secret codebooks which soon offered invaluable clues as the plans and movements of the German High Seas Fleet. Before long, the codebooks used by German warships, U-boats and naval Zeppelins, as well as the ciphers used by the Germans to communicate with their naval attaches and embassies, had been broken by British naval and military codebreakers. Paul Gannon’s vibrant account, based on previously secret files, brings to life hidden stories of British codebreakers in World War I who worked inside Room 40 and its secretive military counterpart, MI1(b). He recounts how intercept intelligence ensured British dominance of the seas, brought the US into the war, won the ‘First Battle of Britain’, changed the military balance in the Middle East and neutered Germany’s international espionage and sabotage operations. Gannon also reveals how extensively Britain listened into the diplomatic communications of neutrals and how codebreaking machinery was first used more than 20 years before its better known appearance at Bletchley Park during World War II. Gannon vividly portrays the brilliant eccentrics who broke tens of thousands of German naval, military and diplomatic messages. This groundbreaking account will change our understanding of how the war was fought and won.