Columbia subject headings:
Armed merchant ships--History--20th century.
Merchant marine--Germany--History--20th century.
Naval convoys--History--20th century.
World War, 1939-1945--Aerial operations, British.
Harvard subject headings:
Armed merchant ships--History--20th century.
Merchant marine--Germany--History--20th century.
Naval convoys--History--20th century.
World War, 1939-1945--Aerial operations, British.
Princeton subject headings:
Armed merchant ships--History--20th century.
Merchant marine--Germany--History--20th century.
Naval convoys--History--20th century.
World War, 1939-1945--Aerial operations, British.