Columbia subject headings:
Espionage--Communist countries--History--20th century.
Espionage--Soviet Union--History--20th century.
Espionage--United States--History--20th century.
Spies--United States.
Harvard subject headings:
Espionage, Soviet--United States--History.
Espionage--Communist countries--History--20th century.
Espionage--Soviet Union--History--20th century.
Espionage--United States--History--20th century.
Spies--United States
Princeton subject headings:
Espionage--Communist countries--History--20th century.
Espionage--Soviet Union--History--20th century.
Espionage--United States--History--20th century.
Spies--United States.