Columbia subject headings:
Harvard subject headings:
Fortification--Germany--History--20th century--Congresses.
Germany--History--1933-1945--Congresses.
National socialism--Congresses
Nature conservation--Germany--Siegfried Line--History--20th century--Congresses.
Siegfried Line (Germany)--History--Congresses.
Princeton subject headings:
Fortification--Germany--History--20th century--Congresses.
Germany--History--1933-1945--Congresses.
National socialism--Congresses.
Nature conservation--Germany--Siegfried Line--History--20th century--Congresses.
Siegfried Line (Germany)--History--Congresses.