ected in one place and dependent for existence upon
the labour of others and the supreme importance of rapidity--which between
them render obstacles that seem indifferent to a civilian in time of peace
so formidable to a General upon the march.

The heavy train, the artillery, the provisioning of the force, can in
general only proceed upon good ways or by navigable rivers. At any rate,
if the army departs from these, a rival army in possession of such means
of progress will have the supreme advantage of mobility.

Again, upon the flat an army may proceed by many parallel roads, and thus
in a number of comparatively short columns, marching upon one front
towards a common rendezvous. But in hilly country it will be confined to
certain defiles, sometimes very few, often reduced to _one_ practicable
pass. There is no possibility of an advance by many routes in short
columns, each in touch with its neighbours; the whole advance resolves
itself into one interminable file.

Now, in proportion to the length of a column, the units of which must each
march one directly behind the other, do the mechanical difficulties of
conducting such a column increase. Every accident or shock in the long
line is aggravated in proportion to the length of the line. Finally, a
force thus drawn out on the march in one exiguous and lengthy trail is in
the worst possible disposition for meeting an attack delivered upon it
from either side.

All this, which is true of the actual march of the army, is equally true
of its power to maintain its supply over a line of hills (to take that
example of an obstacle); and therefore a line of hills, especially if
these hills be confused and steep, and especially if they be provided
with but bad roads across them, will dangerously isolate an army whose
general base lies upon the further side of them.

What the reader has just read explains the peculiar character which the
valley of the Upper Danube has always had in the history of Western
European war.

The Rhine and its tributaries form one great system of communications,
diversified, indeed, by many local accidents of hill and marsh and forest,
with which, for the purposes of this study, we need not concern ourselves.

In a lesser degree, the upper valley of the Danube and its tributaries,
though these are largely in the nature of mountain torrents, forms another
system of communications, nourishing considerable towns, drawing upon
which communications, a