The fascinating story of 20th-century efforts by all sides to break down the enemy's signals security, to protect the sender's signals, and penetrate the enemy's secret communications. Many battles in the last 100 years have been won or lost because one side or the other could read its opponent's intercepted messages. From the famous Zimmerman Telegram in April 1917 through to the breaking of the Japanese Purple codes and the more recent Russian codes, trace the history of code breaking.