This book traces the development of American radio intelligence from its beginnings in World War I through the end of World War II and reveals that signals intelligence is useful to the tactical and operational level commander. The book covers the initial appearance of radio intelligence units on the battlefields of France in the first world war, identifying instances where radio intelligence played a role in command decisions. The development of training and doctrine in the period between the two world wars is examined with the organization, doctrine, and training of radio intelligence units as they prepared for combat. It provides a glimpse into the intelligence support provided to the corps, army, and army group commanders during World War II through examination of actual intercept operations and where possible the study compares and contrasts German radio intelligence units and operations with their American counterparts.