Columbia subject headings:
Austria--Relations--Bulgaria--History--20th century.
Bulgaria--History--1878-1944.
Bulgaria--Politics and government--1918-1944.
Bulgaria--Relations--Germany--History--20th century.
Germany--Relations--Bulgaria--History--20th century.
Harvard subject headings:
Espionage, Austrian--Bulgaria--History--Sources.
Espionage, German--Bulgaria--History--Sources.
World War, 1914-1918--Secret service--Bulgaria.
World War, 1939-1945--Secret service--Bulgaria.
Princeton subject headings:
Espionage, Austrian--Bulgaria--History--Sources.
Espionage, German--Bulgaria--History--Sources.
World War, 1914-1918--Secret service--Bulgaria.
World War, 1939-1945--Secret service--Bulgaria.