Columbia subject headings:
Plenge, Johann, 1874-1963.
Socialism--Germany.
World War, 1914-1918.
Harvard subject headings:
Plenge, Johann, 1874-1963.
Socialism--Germany--20th century.
Socialism--Germany--History--20th century
Socialists--Germany
Princeton subject headings:
Plenge, Johann, 1874-1963.
Socialism--Germany.
World War, 1914-1918.