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.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.