system of defense structures for protection from enemy attacks. Fortification developed along two general lines: permanent sites built in peacetime, and emplacements and obstacles hastily constructed in the field in time of war.
As long as weapons remained relatively primitive, permanent fortifications predominated. The art of fortification developed in earliest times with the building of earthworks made up of layers of mud, sticks, rocks, and the like. These soon were developed into walls, then into palisades and elaborate wooden stockades. In the Middle East walled cities appeared very early. Those of Mesopotamia had walls of mud or sun-dried brick built to withstand invaders. The citadel, a fort or fortified section within the city, also appeared early. Phoenician cities were strongly walled and offered sturdy resistance to Assyrian, Persian, and Macedonian attackers. Major developments in permanent fortification were made by the Romans, who constructed walls along the Danube and Rhine and in England (e.g., Hadrian's Wall). Some of these had elaborate systems of watchtowers, with provisions for garrisoning men along the walls. In E Asia the famous Great Wall of China was an even more ambitious undertaking of the same type.
To overcome advances in fortification, siegecraft (see siege) evolved, and devices such as battering rams, scaling ladders, catapults, and movable towers appeared. As siegecraft became more effective, walls were made higher and thicker—often 30 to 40 ft (9.1–12.2 m) thick. The Romans, with their engineering skill, also developed field fortifications in their camps. However, with the breakdown of Roman authority and the increase in raids and incursions by invaders from the North and the East, fortification on the grand scale was largely replaced by local fortifications.
In the Middle Ages, when raids and petty warfare were customary, the typical fortifications were town walls of masonry, great citadels within the cities, and castles. The Crusades helped further the development of fortifications. Similar structures were used in the chaotic warfare of feudal China, India, and Japan. In the West many castles and citadels, notably those of the Moors in Spain, were defensible against all but a long siege.
The development of artillery in the 15th cent. greatly diminished the value of medieval castles. One of the great military problems of the Renaissance and the succeeding centuries was to develop fortifications able to withstand artillery. Moats were deepened to afford greater protection and widened to put artillery at a greater distance. Walls were lowered, thickened, slanted, and rounded to resist projectiles and make them ricochet, and stone bulwarks were thrown up in front of towers and gates. New fortifications, set in ditches, were buttressed to withstand heavy shot, and defensive guns were mounted behind earthen ramparts. In fortifications of towers (roundels) connected by walls (curtains), there were areas that could not be covered by defensive fire from the towers. Hence artillery positions, or bastions, were constructed at angles to the main wall. The proper distribution of bastions became the main preoccupation of military engineers.
The science of military engineering reached a high point in the wars of Louis XIV. Sébastien le Prestre, marquis de Vauban, who worked out fortification and siege methods in the late 17th cent., has perhaps the most illustrious name in the history of fortification. His methods, supported by the work of others such as Menno van Coehoorn, were used for centuries.
On the American frontier semipermanent forts and stockades were built in large numbers as garrisons for troops engaged in Indian wars and as refuges for settlers. The Native Americans built forts as well. In Europe the detached fort as a support for outer defense of the fortress chain was introduced to create an entrenched camp between the citadel of the fortress and the outer edges of the defended area. The trend toward spreading the chain of defense (the enceinte) was hastened in the 19th cent. by the development of explosive shells and more effective artillery. In the second half of the 19th cent., lines of smaller forts and entrenched camps, connected by perimeter railroads, were used to encircle cities and guard strategic points on frontiers. Batteries were dispersed, artillery was placed in revolving or disappearing cupolas with subterranean bases, and pillboxes, armed with machine guns, were introduced.
This system was predominant in Europe at the beginning of World War I. However, the Belgian fortresses, which had been thought impregnable, fell with ease to the Germans in 1914, and the ring system of fortification was generally superseded during the war by trench warfare. The resistance of French concrete forts, even to the heaviest fire, seemed to offer a promise of permanent defensive fortification and inspired the construction of the Maginot Line. That elaborate system of pillboxes, forts, and underground communications was constructed at great expense.
At the beginning of World War II, the Maginot Line was quickly outflanked (May, 1940). The development of airpower, heavy artillery, and mechanized warfare further proved the inefficacy of such massive defensive systems and brought them to an end. Despite the value of the German Siegfried Line, which long withstood heavy assault in 1944, and despite the usefulness of the Stalin line in channeling the German attack on Russia, field fortifications predominated over fixed fortifications in World War II. However, underground shelters were used for protection from air attack, and the Germans constructed large concrete shelters to protect submarines in harbor. The Japanese fortified Pacific islands with caves and with simply constructed pillboxes and bunkers. Similar fortifications were used in the Korean and Vietnam wars. The last years of the Korean War were virtually trench warfare. In Vietnam, the Viet Cong perfected underground complexes in the field, whereas the United States built a network of installations and artillery firebases protected by air forces and the usual land defenses.
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