estimated by the best military writers, that an army supported by a
judicious system of fortifications, can repel a land force _six_ times
as large as itself.

Every government should prepare, in time of peace, its most prominent
and durable means of defence. By securing in a permanent manner its
important points, it will enable a small force to retain possession of
these places against a greatly superior army, for a considerable length
of time. This serves the same purpose as a battle gained; for, in the
beginning of a war of invasion, the economy of time is of the utmost
importance to the defensive party, enabling it to organize and prepare
the great military resources of the state.

In all mountainous frontiers, or sides of states bordering on large
rivers, or chains of lakes, there will necessarily be but few points by
which an invader can penetrate into the interior of the country. Let us
suppose that, for a frontier of moderate extent, there are _five_
passes, or avenues through which the enemy may approach the interior. To
effectually defend these approaches against the invading army will
require, for each, an army of ten thousand men. Not being able to decide
positively on the plans of the enemy, all these communications must be
defended at the same time. This requires a defending army of fifty
thousand men. Let us now suppose each of these passes to be fortified in
such a way, that one thousand men will be able to hold the enemy in
check, and force him to resort to the operations of a siege; or, at
least, to retard his advance till an active army can be organized in the
interior, and prepared to meet him in the field. We here see that five
thousand men, by means of fortifications, can accomplish the same
defensive object as fifty thousand men without these artificial means of
security.