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had been thought impregnable, fell with ease to the Germans in 1914, and the ring system of fortification was generally superseded during the war by trench warfare.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.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.