An analysis of the insider information and insights that over eighty Soviet intelligence officer defectors revealed during the first half of the Soviet period Identifies 88 Soviet intelligence officer defectors for the period 1917 to 1954, representing a variety of specializations; the most comprehensive list of Soviet intelligence officer defectors compiled to date. Shows the evolution of Soviet threat perceptions and the development of the "main enemy" concept in the Soviet national security system. Shows fluctuations in the Soviet recruitment and vetting of personnel for sensitive national security positions, corresponding with fluctuations in the stability of the Soviet government. Compiles for the first time corroborative primary sources in English, Russian, French, German, Finnish, Japanese, Latvian, Norwegian, Portuguese, Spanish, and Swedish.When intelligence officers defect, they take with them privileged information and often communicate it to the receiving state. This book identifies a group of those defectors from the Soviet elite - intelligence officers - and provides an aggregate analysis of their information to uncover Stalin’s strategic priorities and concerns, thus to open a window into Stalin’s impenetrable national security decision making. This book uses their information to define Soviet threat perceptions and national security anxieties during Stalin’s time as Soviet leader.