Siege (Fr., 'a seat,' 'a sitting down'). When the assault of a fortified place would be too hazardous and costly and its reduction by blockade too slow, recourse is had to the regular siege or systematic attack. In order to cross the open ground swept by the fire of the fortress with as little loss as possible the besieger makes use of sunken roads or trenches. The revetments having been breached by his artillery or mines, he continues these roads through the breaches into the place. To prevent these approaches being enfiladed by the guns of the fortress they are made at first in zigzags; the prolongations of which are directed so as to clear the works of the fronts attacked, and, when a direct advance becomes necessary, they are provided with traverses at short intervals, or blinded sap is used—i.e. a trench covered in with timber and earth. Two or three such lines of approach are used. To protect and connect them lateral trenches are formed from which large bodies of troops can fire upon any sortie that may be made. These are termed parallels, being parallel to the general front of the parts attacked. The first parallel is made at as short a distance as possible (perhaps 1000 yards) from the fortress, and its construction follows the disorganisation of the defenders' artillery by that of the attack from its first position, often 4000 yards from the fortress; the second would be rather less than midway between the first and the most advanced works of the fortress, so that the supporting troops in it may be nearer the working parties in the approaches than the enemy; the third parallel would generally be about 100 yards in front of the salients of the covered way. Parallels are useful also to connect the batteries which, dispersed over a wide area, will concentrate their fire upon the revetments of the ditch and ramparts, or upon the guns of the defence. These latter, restricted as they are to a comparatively small space, must eventually be outnumbered and overpowered; but, if the investment is imperfect, as at the siege of Sebastopol in 1854-55, an active defender may long delay this disaster, especially if the fortress has an outer girdle of detached forts, for in that case at least two of the forts must be taken by regular siege before further advance is made, and the ground between them (1000 yards and upwards) lends itself to the construction of new batteries to meet those of the attack. But the resources in men, guns, and ammunition must, by the nature of the case, be largely in favour of the attacker, and therefore, if persevered in, the siege is sure to succeed.
The ancients used to surround the place attacked with a high bank of earth, called a line of circumvallation, and protected themselves against attack from the outside by another called a line of contravallation, and a similar arrangement was in vogue until the middle of the 19th century. Now a covering field-army is employed, which, by its greater mobility, is able to meet the relieving army many miles from the besieging force, and a chain of fortified localities takes the place of the continuous line of circumvallation.
In order that a siege may be safely undertaken the strength of the besieger should be about four times that of the garrison. Thus, in the case of a small place with a garrison of 5000 men the line of investment would probably be 12 miles long, and could be maintained by three detachments of 2500 men each, the guards of the trenches would be 5000 and working parties 8000—a total of 20,500. In 1870 Strasburg, with a garrison of 20,000, was captured by a besieging force 60,000 strong. Metz was starved into surrender, the presence of so many men (some 170,000) besides the proper garrison only hastening that result. Had this large entrenched camp been held by its regular garrison of 40,000 men it would not have fallen to a besieger with less than 120,000, which is more than half the numbers actually employed.
The siege trains employed in 1870 by the Germans were comparatively small, owing to the badly prepared state of the French fortresses. Thus, at Strasburg only 243 pieces were used, firing 200,000 rounds in thirty-six days, while at Sebastopol the allies mounted 698 pieces of artillery, and in three days these fired 150,000 rounds.
The siege parks, or main depôts, for the artillery and engineer trains must be out of range of the enemy's guns, containing as they do powder, ammunition, guns, and warlike stores of all descriptions. The batteries necessary are enflade batteries, placed on the prolongations of all the important works attacked; counter batteries, to overcome the fire of the works bearing upon the field of attack; mortar and howitzer batteries, to search by high-angle fire the interior of all the works attacked; and breaching batteries, to breach by curved fire the scarps and flanking casemates. Light pieces, such as the seven-pound mountain gun and machine guns, are placed in the second and third parallels, and in the demi-parallels or lodgments, 100 to 150 yards long, made on each approach about half-way between these parallels.
Beyond the third parallel the besieger will probably be met by counter-mines, and himself have to resort to mining in order to carry out the crowning of the covered way. He will then connect his approaches by a fourth parallel, establish batteries and lodgments on the crest of the covered way, and from them mine down to the back of the counterscarp, which he can blow in previous to sapping across the ditch and up the breaches. From the positions thus gained a further advance, if necessary, can be made until the last retrenchment is taken, and the place falls. See also FORTIFICATION, and MINES.
Among great sieges in the world's history may be mentioned those of Troy, Tyre (572, 332 B.C.), Syracuse (396 B.C.), Sagnntum (219 B.C.), Jersalem (70 A.D.), Acre (1191, &c.), Calais (1347), Orleans (1428), Constantinople (1453), Haarlem (1572-73), Leyden (1574), Breda (1625), Rochelle (1628), Magdeburg (1631), Breisach (1638), Tannunton (1644-45), Londonderry (1689), Gibraltar (1731, 1779, 1782-83), Prague (1741-44), Leipzig (1757, 1813), Quebec (1759-60), Seringapatam (1799), Genoa (1800), Saragossa (1808-9), Cindad Rodrigo (1810, 1812), New Orleans (1814), Antwerp (1832), Rome (1849), Sebastopol (1854-55), Kars (1855), Lucknow (1857), Delhi (1857), Gaeta (1860-61), Vicksburg (1863), Charleston (1863-64), Richmond (1864-65), Metz (1870), Strasburg (1870), Belfort (1870-71), Paris (1870-71), Plevna (1877), and Khartoum (1884).
The state of siege as defined by continental jurists is a condition of things in which civil law is suspended or made subordinate to military law. A fortress, city, or district is thus put under martial law—i.e. under the authority of the military power—either on account of the presence of an enemy, as at a siege, or because of the failure of the civil power, as in the case of domestic insurrection, or of a conquered district in military occupation. The minor state of siege, a modification of the more severe rule, usually suffices for domestic troubles. No such provision is made by the laws of the British Empire or of the United States, though very similar powers are exercised when martial law is proclaimed. For this no rules are made—the possibility of civil war is not presupposed; but should the civil power become inoperative it is the duty of the supreme authority to maintain order by any means (usually of course an armed force) that are available, afterwards coming to parliament for an act of indemnity to justify conduct in itself contrary to law. Perhaps an approach to the continental minor state of siege may be found in the restricted power to try offenders in Ireland by military tribunals created by act of parliament in 1799, 1803, and 1833.