Editing Fortification (section)

===20th and 21st centuries===
[[File:Fort Campbell 09.jpg|thumb|Gun emplacement in [[Fort Campbell (Malta)|Fort Campbell]], built in the 1930s. Due to the threat of [[aerial warfare]], the buildings were placed at a distance from each other, making it difficult to find from the air.]]

[[Steel]]-and-[[concrete]] fortifications were common during the 19th and early 20th centuries. However, the advances in modern warfare since [[World War I]] have made large-scale fortifications [[obsolete]] in most situations. In the 1930s and 1940s, some fortifications were built with designs taking into consideration the new threat of [[aerial warfare]], such as [[Fort Campbell (Malta)|Fort Campbell]] in Malta.<ref>{{cite web|last1=Mifsud|first1=Simon|title=Fort Campbell|url=http://www.militaryarchitecture.com/index.php/Fortifications/fort-campbell.html|website=MilitaryArchitecture.com|access-date=15 March 2015|date=14 September 2012|url-status=live|archive-url=https://web.archive.org/web/20151115113234/http://www.militaryarchitecture.com/index.php/Fortifications/fort-campbell.html|archive-date=15 November 2015}}</ref> Despite this, only underground [[bunker]]s are still able to provide some protection in modern wars. Many historical fortifications were demolished during the modern age, but a considerable number survive as popular tourist destinations and prominent local [[landmark]]s today.

The downfall of permanent fortifications had two causes:
* The ever-escalating power, speed, and reach of artillery and airpower meant that almost any target that could be located could be destroyed if sufficient force were massed against it. As such, the more resources a defender devoted to reinforcing a fortification, the more combat power that fortification justified being devoted to destroying it, if the fortification's destruction was demanded by an attacker's strategy. From [[World War II]], [[bunker buster]]s were used against fortifications. By 1950, [[nuclear weapon]]s were capable of destroying entire cities and producing dangerous [[radiation]]. This led to the creation of civilian nuclear [[air raid shelter]]s.
* The second weakness of permanent fortification was its very permanency. Because of this, it was often easier to go around a fortification and, with the rise of mobile warfare in the beginning of World War II, this became a viable offensive choice. When a defensive line was too extensive to be entirely bypassed, massive offensive might could be massed against one part of the line allowing a breakthrough, after which the rest of the line could be bypassed. Such was the fate of the many defensive lines built before and during World War II, such as the [[Siegfried Line]], the [[Stalin Line]], and the [[Atlantic Wall]]. This was not the case with the [[Maginot Line]]; it was designed to force the Germans to invade other countries (Belgium or Switzerland) to go around it, and was successful in that sense.<ref>{{cite book |last=Halter |first=Marc |title=History of the Maginot Line |publisher=Moselle River |date=2011 |isbn=978-2-9523092-5-7}}{{page needed|date=August 2022}}</ref>
[[File:Image-GBU-24 Missile testmontage-gi BLU-109 bomb.jpg|thumb|left|upright|A [[GBU-24 Paveway III|GBU-24]] {{cvt|2000|lb}} bomb hits the ground. The development of [[bunker buster]]s, bombs designed to penetrate hardened targets buried underground, led to a decline in the use of fortifications.]]
Instead field fortification rose to dominate defensive action. Unlike the [[trench warfare]] which dominated World War I, these defenses were more temporary in nature. This was an advantage because since it was less extensive it formed a less obvious target for enemy force to be directed against.

If sufficient power were massed against one point to penetrate it, the forces based there could be withdrawn and the line could be reestablished relatively quickly. Instead of a supposedly impenetrable defensive line, such fortifications emphasized [[defense in depth]], so that as defenders were forced to pull back or were overrun, the lines of defenders behind them could take over the defense.

Because the mobile offensives practiced by both sides usually focused on avoiding the strongest points of a [[defensive line]], these defenses were usually relatively thin and spread along the length of a line. The defense was usually not equally strong throughout, however.

The strength of the defensive line in an area varied according to how rapidly an attacking force could progress in the terrain that was being defended—both the terrain the defensive line was built on and the ground behind it that an attacker might hope to break out into. This was both for reasons of the strategic value of the ground, and its defensive value.

This was possible because while offensive tactics were focused on mobility, so were defensive tactics. The dug-in defenses consisted primarily of infantry and [[antitank gun]]s. Defending [[tank]]s and [[tank destroyer]]s would be concentrated in mobile [[brigade]]s behind the defensive line. If a major offensive was launched against a point in the line, mobile reinforcements would be sent to reinforce that part of the line that was in danger of failing.

Thus the defensive line could be relatively thin because the bulk of the fighting power of the defenders was not concentrated in the line itself but rather in the mobile reserves. A notable exception to this rule was seen in the defensive lines at the [[Battle of Kursk]] during World War II, where German forces deliberately attacked the strongest part of the [[Soviet Union|Soviet]] defenses, seeking to crush them utterly.

The [[terrain]] that was being defended was of primary importance because [[open terrain]] that tanks could move over quickly made possible rapid advances into the defenders' rear areas that were very dangerous to the defenders. Thus such terrain had to be defended at all costs.

[[File:NORADNorth-Portal.jpg|thumb|[[Cheyenne Mountain Complex]] is an underground [[bunker]] used by [[North American Aerospace Defense Command]]. Cheyenne Mountain is an example of a mid-20th century fortification built deep in a mountain.]]

In addition, since in theory the defensive line only had to hold out long enough for mobile reserves to reinforce it, terrain that did not permit rapid advance could be held more weakly because the enemy's advance into it would be slower, giving the defenders more time to reinforce that point in the line. For example, the [[Battle of the Hurtgen Forest]] in Germany during the closing stages of World War II is an excellent example of how difficult terrain could be used to the defenders' advantage.

After World War II, [[intercontinental ballistic missile]]s capable of reaching much of the way around the world were developed, so speed became an essential characteristic of the strongest militaries and defenses. [[Missile silo]]s were developed, so missiles could be fired from the middle of a country and hit cities and targets in another country, and airplanes (and [[aircraft carrier]]s) became major defenses and offensive weapons (leading to an expansion of the use of airports and airstrips as fortifications). Mobile defenses could be had underwater, too, in the form of [[ballistic missile submarine]]s capable of firing [[submarine launched ballistic missile]]s. Some bunkers in the mid to late 20th century came to be buried deep inside mountains and prominent rocks, such as [[Gibraltar]] and the [[Cheyenne Mountain Complex]]. On the ground itself, [[minefield]]s have been used as hidden defenses in modern warfare, often remaining long after the wars that produced them have ended.

[[Demilitarized zone]]s along borders are arguably another type of fortification, although a passive kind, providing a buffer between potentially hostile militaries.