Fortification and Siege Warfare

Examples of the former are city walls, the castles of the Middle Ages, and the frontier fortresses and harbor defenses of relatively modern times. The construction of permanent defensive works implies a continuing requirement for the protection of political, economic, or military interests associated with a particular locality; the development of temporary or field fortification is associated with tactical requirements in the field (see Tactics). Examples include the palisaded camps built by the Roman legions for overnight halts, the pointed wooden stakes used by English foot soldiers to throw enemy cavalry into confusion under arrow fire, and the use of trenches and sandbags in more modern warfare. When a moving situation becomes stabilized, a hastily occupied defensive position may have to be developed into a more strongly fortified position; the classic example of such a development is the stabilization of the western front in Europe during World War I, when trenches and other temporary defense works became permanent fortifications.

The attack on and defense of masonry structures–whether city walls, isolated forts and castles, or extensive barriers such as the Median Wall between the Tigris and Euphrates rivers in Mesopotamia, the Great Wall of China, or the variety of frontier walls built by the Romans against barbarian attack–involved three basic concepts that remained unaltered until the age of gunpowder. In order to reach the defenders, the attackers had to scale the wall, break a passage through it, or burrow underneath it. The techniques of siege warfare were directed toward the accomplishment of these aims, singly or simultaneously, and the techniques of fortification were aimed at preventing such accomplishment.

These opposing techniques interacted, with fortifiers seeking to build the impregnable fortress while besiegers strove to develop the irresistible siege; the effectiveness with which these aims were pursued varied widely through history. Natural defensive strength was the criterion for selection of city sites. In Greece, for example, the huge rock of the Acropolis was the location of the earliest Athenian settlers; the Seven Hills of Rome amid the marshes of the Tiber River provided a strong defensive situation for that city. The Phoenicians of the ancient city of Tyre, finding their coastal position too exposed to marauders, moved to an offshore island and added a water barrier to their defenses. The art of fortification developed through local necessities and, as the wealth and power of the cities grew, often became undermined by complacency. The art of attack by siege, on the other hand, was stimulated by the efforts of those who would be conquerors.

In former times, setting siege to an enemy stronghold demanded enormous labor. Movable wooden towers, or belfroi, were built, from which bows and arrows or slings could be aimed at the garrison, and from which the top of the enemy wall could be attacked. Such towers were of enormous weight; one used at the siege of Rhodes by Demetrius I, king of Macedonia, in 305BC required 3400 warriors to move it. Soldiers in the lower stories of the tower attacked the wall with battering rams (see Battering Ram), consisting of tree trunks swung by ropes from overhead beams, with rounded metal heads to crumble the masonry, or with borers, which were similar devices with ax-shaped heads to attack crevices and to wrench stones out of the face of the wall. The defenders, in turn, would normally attack the wooden tower with fire, against which raw animal hides were the usual method of protection.

Missile-throwing engines included several types of catapult, one of which was a giant crossbow attached to a timber stand. Another kind of catapult, the ballista, used the torsion of heavy cords twisted between two uprights to throw stones weighing up to 45 kg (up to 100 lb) each, although with little accuracy.

The development of artillery revolutionized fortification and siege techniques. In 1453 the enormous walls of Constantinople, the last seat of imperial Roman power, were breached by the guns of the Ottoman sultan Muhammad II. In the same year, French artillery defeated the last English army remaining on French soil and brought the Hundred Years' War to an end. After 1453 fortifiers found that earthwork constructions proved a far more reliable protection against cannonballs than masonry. The ditch therefore became a basic feature in fortification, and siege artillery became the principal means of attack. The ditch usually had a gently sloping approach, called the glacis, swept by the fire of the defenders from a rampart behind the ditch, which was just high enough to command the glacis.

The durability of Vauban's influence may partly account for the relative lack of novelty in the development of fortification and siege techniques during the 18th century. The next really substantial change was brought about by weapons development following the Industrial Revolution. Rifled artillery with longer range and greater accuracy and explosive shells of greater destructive power demonstrated the vulnerability of existing permanent fortifications when the French cities of Metz, Sedan, and Paris were taken during the Franco-Prussian War of 1870-71. Field fortification, however, in association with the fire and movement of rifle-armed infantry, was beginning to come into its own, as the events of the American Civil War (1861-65) had demonstrated.

The fortress of West Point, New York, which had been a strategic pivot of operations during the American Revolution, became in 1802 the seat of a military academy that worked on developing the arts of fortification and siege warfare. Based on Vauban's theories, the American approach to fortification envisaged harbor defense as the principal function of permanent works. Field fortifications were developed for use in mobile situations. Infantry armed with the new muzzle-loading rifles could use field fortification to establish a pivot of maneuver with a comparatively small proportion of the available infantry strength, while a high proportion could be used for the maneuver force. The early Confederate victories in Virginia, including the Battle of Chancellorsville (see Chancellorsville, Battle of), owed much to the daring application of this tactical device by General Robert E. Lee. When a running battle came to a standstill, as at the siege of Vicksburg (see Vicksburg, Campaign of) in 1863 or Petersburg in 1864, fieldworks tended to develop into semipermanent fortifications. The permanent harbor fortifications in the zone of operations played a relatively minor part in the Civil War; the seaward defenses of several Southern ports were reduced by amphibious operations of the Union troops.

The stabilized situation of the Western Front in Europe during World War I was the result of a relatively short front with no open flanks. Tactically, the enormously increased volume of fire of automatic rifles and machine guns gave a decisive advantage to defending infantry, protected by fieldworks and barbed-wire obstacles. The opposing armies were sunk into the earth in trenches, where they could manage to muster enough strength to repel an assault from infantry advancing over open ground. After a 3-year bloody deadlock, it seemed that the ideal of the fortifier's art, the impregnable fortress, had been attained. The deadlock was broken only when armored vehicles, combining firepower and mobility with protection, came into use. During World War I, air warfare–chiefly valuable at this time for reconnaissance and for directing and adjusting artillery fire–was also initiated. This development emphasized the value of concealment and deception, and added scientific camouflage as an essential element of the art of fortification. Initially, permanent fortifications in France and Belgium proved valuable chiefly in delaying actions, although such works were later incorporated, as at Verdun, into general schemes of defense. On the Russian front and in the Middle East, the importance of fortification was proportionately reduced by the magnitude of the areas involved, and a war of movement prevailed.

In the Pacific campaign, the Japanese surprise raid on Pearl Harbor emphasized the new vulnerability of seaward defenses against air attack. Land and air operations were subsequently directed against Japanese fortified positions, the most extensive of these being on the island of Okinawa. During the reconquest of the Philippines in 1945, the Japanese defense of the built-up section of the chief seaport, Manila, involved siege warfare and house-to-house fighting similar to that in Stalingrad.