This book is part of the Army historical community's commemoration of the fiftieth anniversary of World War II. American victory in that conflict was brought about not only by the valor of our fighting men and the immensity of our productive capacity, but also by the availability of superb military intelligence. Much of this intelligence came from the ability of our armed forces to intercept and decipher the most secret communications of their adversaries. For many years security considerations prevented any public mention of these successes in the official histories. Now much of the story can be told. To preserve the memory of the Army's role in this intelligence war, the U. S. Army Center of Military History has joined with the History Office, U. S. Army Intelligence and Security Command (INSCOM), to publish this collection of documents on Army signals intelligence in World War II. INSCOM carries on the heritage of the Army's World War II Signal Security Agency, which by breaking the Japanese diplomatic ciphers and military codes helped speed the way of our forces to victory. The book is intended both for an Army audience and for the general public - including those World War II veterans who participated in the signals intelligence war and who for so many years were constrained to keep their contributions secret. The security barriers have now been lifted, and the Army is proud to acknowledge those contributions. Harold W. NelsonCharles F. Scanlon Brigadier General, US ArmyMajor General, US Army Chief of Military HistoryCommanding General, US Army Intelligence and Security Command