FIRST PERIOD.
69. As a preliminary to the siege of any fortified place, all possible information is obtained as to the strength and character of its fortifications, the garrison, armament, stores of provisions and ammunitions, water supply, water routes, telegraph and railroad lines, manufactures, especially those which may be converted into factories of arms and munitions, the character of the population of the place, their probable food supply and their loyalty to their state; also the topographical features and nature of the ground in the vicinity of the work, the sites of camps and parks, the prevalent diseases of the locality and the best means of preventing their attacks, etc., etc. (see Bureau of Intelligence, Art of War, par. 128).
From these data the necessary materials and supplies are collected at convenient points, the railroad or water routes selected, and the cars, boats, wagons, etc., for their transportation provided; so that they may arrive promptly and in the proper order when needed.
70. The Investment.—The investing force is brought together, organized, and moved rapidly upon the place. When it is available a large force of mounted troops may be used advantageously in the investment, and be subsequently relieved by infantry and artillery.
When the investment is made, it adds greatly to the advantage of the attack to completely surround and isolate the work, and to push the investing line as near it as possible. When the investing force is more or less dispersed, and is to be concentrated for the siege, the temptation frequently exists to march them by converging lines upon the place as a point of concentration.
While this may be advisable in some cases (as where the garrison is very weak or under an inefficient commander), it will usually expose the subdivisions of the investing force to be beaten in detail (Art of War, par. 392). So also in surrounding the place; a premature subdivision of the force into small fractions not protected by field-works, or not within supporting distance each other, will afford to an active defence an opportunity, by well-conducted sorties, to inflict most severe losses upon the attack and very greatly delay the investment. Keeping these dangers in view, the investing force will move rapidly upon the work, seize, strengthen, and occupy strong points as near the work as possible, and extend the lines to right and left as rapidly as good judgment allows, until the place is surrounded. Meanwhile detachments of greater or less size will scour the ground around the place, seizing and carrying off or destroying, so far as possible, all cattle, grain, lumber, etc., and everything else which would be of use to the attack or defence. Under cover of these detachments and escorts specially detailed for the purpose, reconnoissances will be made to cover so much of the ground as can be reached, especial efforts being made to examine the ground near the works. These reconnoissances will necessarily be hurried and incomplete but, must be as accurate as they can be made under the circumstances. They should be directed principally to determining the heights and directions of the principal points of the works, and their positions with reference to prominent points that may be used as landmarks, in verifying and correcting maps and information previously obtained, to discovering the existing armament of the place and the steps already taken for its defence, and to collecting all possible information bearing upon the selection of the front of attack.
Systematic reconnoissances and surveys carried on throughout the siege must be relied upon for checking and completing the work thus begun.
So soon as the supporting points for the investing force are secured, a line of outposts is pushed forward towards the work and sentinels, pickets, etc., are established (Art of War, pars. 167-194). The lines of sentinels, pickets, and supports are placed as near the work as practicable, and the line of resistance is advanced at every favorable opportunity.
The usual rules for posting and relieving the outposts, establishing day and night cordons, the use of patrols, etc., are applied, with such modifications as circumstances render advantageous.
Any advanced points affording marked advantage to the attack which have been seized are strengthened and held when possible, even at considerable cost in men or with some delay in completing the investment.
71. Bringing up and Posting the Besieging Force.—The main besieging force, consisting principally of infantry, artillery, and engineers, with the siege train, follows closely after the investing force, and, upon arrival is encamped upon sites previously selected, sending out at once, however, such reinforcements and supports as are needed by the line of investment. Engineer and artillery parks are established outside the zone of fire of the works and in proximity to the main routes of communication. Branch railways and tram-roads running through the parks, storehouses, repair shops, etc. etc., are located and constructed. Sites for storage magazines for ammunition are carefully selected at the most secure places, and isolated when possible from the camps and parks by intervening elevations of ground. The cover of these magazines, so far as possible, is made up of wood and sand or earth free from stones large enough to be dangerous projectiles in case of explosion. Rooms for loaded shells and cartridges, and laboratories for making up ammunition are constructed upon similar principles. Carefully studied arrangements for the health and comfort of the men are made. Some of these are outlined in Chapter VII.
72. Fortifying the Camps, Parks, etc., etc.—In former sieges it was customary to completely surround the ground occupied by the besieger with a continuous line of works of simple trace and light profile called the “line of circumvallation;” and to construct between the camps and the work another line, either continuous or with intervals, called the “line of countervallation.” These lines were placed respectively at about 200 yards in rear and in front of the camps.
The principal object of the first was to prevent by a small force the entry of small reinforcing detachments and supplies; that of the second was to resist vigorous sorties by the defence, or sudden attacks from the outside by strong reinforcing parties. For this purpose the detached works of the line of countervallation were so disposed as to cover the main depots, parks, roads, etc., and to be in defensive relations with each other.
The great development of the line which must be occupied by the besieger, owing to modern methods of fortification and the range of rifled cannon, prohibits the construction of complete lines of circum- and countervallation. The besieger constructs in their stead one or more lines of detached works upon advantageous points, and covers the intervening ground more or less thoroughly by patrols, outposts, etc. He then so disposes his main force as to be able to concentrate enough to meet any sorties of the defence; and, if necessary, detaches a force, called an “army of observation,” sufficiently large to meet any relieving army and defeat it; or hold it in check until he can concentrate the besieging force with the army of observation, and meet the relieving army in a favorable position. As a rule, this position will be one well outside the besieger’s cordon of works; since the latter by its extent will necessarily be weak to resist a determined attack (Art of War, par. 258), and by its proximity to the work will render possible the co-operation of the garrison and the relieving army. This, under the circumstances assumed, would seriously endanger the besieging army.
In opposing sorties from the work, however, the conditions which fix the point of conflict are reversed, and place it as near the work as practicable. The shortening and strengthening of the line of investment by closing it in upon the work make it imperative to hold all ground gained; and this is generally best accomplished by intrenching the line of outposts with continuous shelter trenches, strengthened at intervals by batteries of field guns, and supported by field works of considerable strength, placed within accurate cannon range of each other, but not exposed to the direct fire of the guns of the place. Behind the shelter trenches the outposts, supports, and reserves, strengthened when necessary by troops from other points of the line, should be able to hold their own against all ordinary sorties. The main line of field works serves to resist a general attack made by the mass of the garrison.
Placing the first intrenchments further back exposes the outposts to the confusion resulting from falling back, frequently at night or in a fog, and also enables the besieged to seize upon ground from which it may be very difficult to dislodge them.
To allow the different parts of the line to be rapidly reinforced, good roads protected from the fire of the work, and well marked with sign-posts, etc., must be opened between the adjacent divisions of the besieging force, and all streams must be provided with bridges secure against floods, ice, etc.
73. Distance of the Line of Investment from the Work.—This will result from conflicting conditions. Reasons already given, which need not be repeated, lead to establishing it as near as practicable. On the other hand, the accurate fire of the heavy guns of the place, and vigorous sorties by the defence, cause much annoyance and great loss to a line drawn too near the work. The more recent sieges indicate about 3000 yards from the most advanced works, as the least distance for the line of investment in open country and with an active defence. It may be necessary in some cases to increase this to 4500 or 5000 yards; but with ground favorable to the attack, and a weak and demoralized defence, it may frequently be drawn nearer.
74. Strength and Composition of the Besieging Force.—In former sieges when the place held out until the inner keep was breached and carried by the regular progress of the siege, the ratio of the necessary strength of the attack to the defence was estimated at 7 or 8 to 1, this large ratio resulting from the excessive labor in the trenches and the losses incurred on the close attack. Modern writers (arguing largely upon theoretical considerations) have reduced this estimate to 4 or 5 to 1. No attack on a thoroughly-equipped and well-defended strong place having been carried through all the steps of a regular siege since the introduction of modern arms, absolute data upon this subject are lacking.
The besieging force at Strasburg was about 60,000, garrison about 20,000, total length of siege 49 days. The defence was very weak. Belfort, besieging force about 32,000, garrison about 16,000. After a siege of 100 days the approaches were at about 1200 yards from the works, which capitulated by reason of the general surrender of the French. At Metz the besieging force was 150,000 men; the garrison, demoralized by the previous defeat at Gravelotte, surrendered 173,000 men. At Paris the investing force was about 180,000, and the garrison nominally between 300,000 and 400,000, of which perhaps 30,000 were disciplined and effective soldiers; the remainder being made up of remnants of defeated regiments and bodies of the Garde Mobile and Garde Nationale. The investment of Paris was complete on September 19, 1870; its surrender from exhaustion of provisions took place January 29, 1871. Several sorties were made, but the general defence was paralyzed by the character of the troops and inhabitants. At Plevna the Turks had at the outset about 56,000 men, at the surrender 40,000. The Russian force suffered great losses in its assaults, but by continual reinforcement had at the end of the siege about 120,000 men. The defence by the Turks was desperate, but generally passive. One determined sortie was made immediately before the surrender. The surrender resulted from exhaustion of ammunition and provisions. The works were field works only.
At Belfort the investing force was at first but 10,000 and the line of investment 25 miles long, giving but 400 men to the mile. This force was subsequently increased to 20,000 men, and when the besieging army had all arrived, to 32,000 men.
At Paris (1870) the line of investment was about 3 miles from the line of the forts, and about 53 miles in length, the investing force 180,000, giving a mean of about 2 men to the yard. The distribution was, however, about 4 to the yard on the left bank and 1⅓ on the right bank of the Seine.
At Plevna the line of investment was 2¾ miles from the forts, its length 43½ miles, the investing force 100,000 men, about 1¼ men to the yard.
In each of these sieges the place finally fell under the attack of a force, in no case equal to 2½ times the garrison; but inferences drawn from this fact are apt to be erroneous, since none of these places was well fortified according to modern methods, well garrisoned, well supplied, and defended to extremity.
The results show, however, that under similar circumstances, which are apt to arise in any modern war, the attack of a strong place which can be completely invested by a force of two or three times the strength of the garrison, may promise success; which seems to be assured if the defence allows the besieging force to complete the investment and thoroughly intrench itself.
On the other hand, tactical considerations would indicate that a well-equipped army, of good morale, under an active and aggressive commander, covered by a modern intrenched camp, should be able to prevent the investment; and by taking advantage of its interior lines, its heavy guns and its strong points d’appui, should be able to beat in detail a force very much greater than itself whose fractions, by reason of the extent of the line of investment, are necessarily not within supporting distance of each other.
These advantages of the defence evidently disappear, as above indicated, when the attack is allowed to complete its fortifications, since under their cover a small force can check even a determined sortie until a sufficient force to beat it can be concentrated.
From these considerations it is evident that an investment, once completed, may be maintained by a force less than that necessary to establish it in the first place (see Investment of Plevna, Pierron, Méthodes de Guerre Vol. III, pp. 647 et seq.).
75. The Point of Attack.—From the information originally in possession of the besieger, supplemented by that obtained by reconnoissance, a decision is made as to the fronts of the work or the particular detached works of the intrenched camp upon which the approaches are to be made. The portion selected in either case is called the “point of attack.” To reduce an intrenched camp, it will in general be necessary to capture at least two of the detached works and to silence the artillery fire of one or more on each side of those taken. In an attack upon a strongly-fortified enceinte, the least that is usually undertaken is to breach and capture one front with its adjacent outworks, and to silence the fire of those which enfilade the approaches and parallels or take them in reverse.
In selecting the point of attack the first consideration is, that when taken, it shall afford material advantage to the besieger and give him a foothold from which further approaches may be driven, if necessary. This condition being fulfilled, the choice will result from a careful study of the nature of the works and site. Those forts or fronts resting upon precipices, bordering deep marshes or deep and rapid streams, or which are so placed that approaches upon them will be swept in flank and rear by the fire of the works, which cannot be silenced, are considered impregnable by the ordinary operations of the siege. Most serious difficulties are presented by those in which the adjacent works are so disposed and of such strength that they can be carried only in succession and by regular approaches; those provided with wet ditches in which strong currents can be produced, those with dry deep ditches, those which are mined, and those which present long lines nearly straight, or even concave to the attack, and covering a front nearly equal or even greater than can be occupied by the trenches of the besiegers.
When the parallels and approaches have to be constructed upon ground sloping downward towards the work, in soil containing large stones, or in which the rock is close to the surface, in marshy ground or that containing much water or liable to be flooded, the difficulty of their construction and defilade are evident. The point of attack considered most favorable to the besieger is one which, fulfilling the first essential condition, is more or less salient, so that it can be partially surrounded, and which admits of the approaches being driven toward it in favorable soil, over ground sloping gently from the work, or gently rolling with the crests and valleys of sufficient difference of level to afford cover, and running generally in the direction of the parallels.
A favorable location for parks, etc., with free, safe, and short communications between them, also has great weight in selecting the point of attack.