Columbia subject headings:
Coal trade--Military aspects--Germany--Ruhr (Region)--History--20th century.
Industrial mobilization--Germany (East)--History--20th century.
Industrial mobilization--Germany (West)--History--20th century.
Industries--Germany--History.
Harvard subject headings:
Coal trade--Germany--Ruhr--Military aspects--History--20th century.
Industrial mobilization--Germany--History--20th century.
Princeton subject headings:
Coal trade--Germany--Ruhr (Region)--Military aspects--History--20th century.
Industrial mobilization--Germany--History--20th century.