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