Columbia subject headings:
Harvard subject headings:
Espionage--Europe--History--20th century.
Intelligence service--Europe--History--20th century.
World War, 1939-1945--Secret service--History.
Princeton subject headings:
Espionage--Europe--History--20th century.
Intelligence service--Europe--History--20th century.
World War, 1939-1945--Secret service--History.