5. The only major natural obstacles to the passage of land forces
into Western Europe from the east are the River Elbe from the Bohemian
border and the River Rhine from Switzerland to the North Sea. Between
these two rivers there are a number of minor obstacles which could be
used for delaying an enemy advance.
It is not considered that the
construction of an artificial defense line across Germany is either
practical or desirable, but considerable advantage may be gained by
the improvement of natural obstacles by artificial means.