In all military operations time is of vast importance. If a single division of an army can be retarded for a few hours only, it not unfrequently decides the fate of the campaign. Had the approach of Blucher been delayed for a few hours, Napoleon must have been victorious at the battle of Waterloo. An equilibrium can seldom be sustained for more than six or seven hours between forces on the field of battle; but in this instance, the state of the ground rendered the movements so slow as to prolong the battle for about twelve hours; thus enabling the allies to effect a concentration in time to save Wellington.
Many of Napoleon's brilliant victories resulted from merely bringing troops to bear suddenly upon some decisive point. Rivoli in 1796-7, Marengo in 1800, Ulm in 1805, Jena in 1806, Ratisbon in 1809, Brienne in 1814, and Ligny in 1815, are familiar examples. But this concentration of forces, even with a regular army, cannot be calculated on by the general with any degree of certainty, unless his communications are perfectly secure. And this difficulty is very much increased where the troops are new and undisciplined. When a country like ours is invaded, large numbers of such troops must suddenly be called into the field. 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. 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.
But let us enter a little more into the details of frontier defences, and examine the character of the several systems which have been successively proposed or adopted. Frontiers are divided into four distinct classes, according as the state may be open on one or more sides, or bounded by mountains, large rivers and lakes, or by the sea.
An open frontier is the most difficult of defence; and while there exists a perfect uniformity among military men upon the vast importance of fortifying such a frontier, there is an equal diversity of opinion respecting the best manner of arranging these works. We shall here mention three general systems of arranging forts for the defence of an open country, each of which has been advocated at different times, and afterwards received various modifications and additions. These three systems comprise the main features of all others worthy of much consideration. They are:—
1st. The system of continuous lines, proposed by Montalembert.
2d. A system of three lines of detached works, strongly recommended by D'Arçon and others.