d it is essential to the
reading of military history to appreciate these difficulties; for the
degrees of impediment which natural features present to thousands upon the
march are utterly different from those which they present to individuals
or to civilian parties in time of peace. Since it is to difficulties of
this latter sort that we are most accustomed by our experience, the
student of a campaign will often ask himself (if he is new to his subject)
why such and such an apparently insignificant stream or narrow river, such
and such a range of hills over which he has walked on some holiday without
the least embarrassment, have been treated by the great captains as
obstacles of the first moment.

[Illustration: Map showing the peril of Marlborough's march to the Danube,
beyond the hills which separate the Rhine from the Danube.]

The reason that obstacles of any sort present the difficulty they do to an
army, and present it in the high degree which military history discovers,
is twofold.

First, an army consists in a great body of human beings, artificially
gathered together under conditions which do not permit of men supplying
their own wants by agriculture or other forms of labour. They are gathered
together for the principal purpose of fighting. They must be fed; they
must be provided with ammunition, usually with shelter and with firing,
and if possible with remounts for their cavalry; reinforcements for every
branch of their service must be able to reach them along known and
friendly (or well-defended) roads, called their _lines of communication_.
These must proceed from some _base_, that is from some secure place in
which stores of men and material can be accumulated.

Next, it is important to notice that variations in speed between two
opposed forces will nearly always put the slow at a disadvantage in the
face of the more prompt. For just as in boxing the quicker man can stop
one blow and get another in where the slower man would fail, or just as in
football the faster runner can head off the man with the ball, so in war
superior mobility is a fixed factor of advantage--but a factor far more
serious than it is in any game. The force which moves most quickly can
"walk round" its opponent, can choose its field for action, can strike in
flank, can escape, can effect a junction where the slower force would
fail.

It is these two causes, then--the artificial character of an army, with
its vast numbers coll