Not knowing the designs of the invaders, much time will be lost in marches and countermarches; and if there be no safe places of resort the operations must be indecisive and insecure.
To a defensive army fortifications are valuable as points of repose, upon which the troops, if beaten, may fall back, and shelter their sick and wounded, collect their scattered forces, repair their materials, and draw together a new supply of stores and provisions; and as rallying points, where new troops may be assembled with safety, and the army, in a few days, be prepared to again meet the enemy in the open field.
Without these defences, undisciplined and inexperienced armies, when once routed, can seldom be rallied again, except with great losses.

But when supported by forts, they can select their opportunity for fighting, and offer or refuse battle according to the probability of success; and, having a safe place of retreat, they are far less influenced by fear in the actual conflict.
The enemy, on the other hand, being compelled either to besiege or _observe_ these works, his army will be separated from its magazines, its strength and efficiency diminished by detachments, and his whole force exposed to the horrors of partisan warfare.

It has therefore been 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.