Haldane (The Hidden World, 1976) worked with ciphers and codes for British Intelligence during World War II. In the present study he confines himself to the successes and failures in the hidden war of secret communications that led to Allied victory. Western security was terribly lax, yet during a single day SHAEF once recklessly enciphered over 9,000,000 words, many of which could have been sent clear text. Early on, the Allies broke the Germans' Ultra and Enigma cipher machines, and the Purple cipher of the Japanese could be read by us even before Pearl Harbor. At the same time home security was so snafued that plans for the Dieppe raid were found in a Whitehall gutter, and a briefcase with the complete D-Day plans was found in a train at Exeter. Churchill believed that our breaking of Ultra and foreknowledge of the enemy's intentions was decisive in our winning the war. Also meaningful was Britain's large-scale turning of German agents into a substantial army of double agents feeding false information back into German intelligence. Breaking the enemy cipher also saved the Battle of the Atlantic in late 1943, with the Admiralty tracking down U-boats as soon as they signaled their positions. The Russians had the largest espionage operation of the War and Haldane attributes their success against Germany specifically to their spies. Richard Serge and Kim Philby receive brief, no-nonsense summaries. Mainly a synthesis, not a personal history, and quite readable with a lightly bitter tone.