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Defence in depth (also known as deep defence or elastic defence) is a
military strategy Military strategy is a set of ideas implemented by military organizations to pursue desired strategic goals. Derived from the Greek word ''strategos'', the term strategy, when it appeared in use during the 18th century, was seen in its narrow sen ...
that seeks to delay rather than prevent the advance of an attacker, buying time and causing additional
casualties A casualty, as a term in military usage, is a person in military service, combatant or non-combatant, who becomes unavailable for duty due to any of several circumstances, including death, injury, illness, capture or desertion. In civilian usag ...
by yielding space. Rather than defeating an attacker with a single, strong defensive line, defence in depth relies on the tendency of an attack to lose momentum over time or as it covers a larger area. A defender can thus yield lightly defended territory in an effort to stress an attacker's
logistics Logistics is generally the detailed organization and implementation of a complex operation. In a general business sense, logistics manages the flow of goods between the point of origin and the point of consumption to meet the requirements of ...
or spread out a numerically superior attacking force. Once an attacker has lost momentum or is forced to spread out to pacify a large area, defensive
counter-attack A counterattack is a tactic employed in response to an attack, with the term originating in " war games". The general objective is to negate or thwart the advantage gained by the enemy during attack, while the specific objectives typically see ...
s can be mounted on the attacker's weak points, with the goal being to cause attrition or drive the attacker back to its original starting position.


Strategy

A conventional defence strategy would concentrate all military resources at a
front line A front line (alternatively front-line or frontline) in military terminology is the position(s) closest to the area of conflict of an armed force's personnel and equipment, usually referring to land forces. When a front (an intentional or uninte ...
, which, if breached by an attacker, would leave the remaining defenders in danger of being outflanked and surrounded and would leave
supply line Supply may refer to: *The amount of a resource that is available **Supply (economics), the amount of a product which is available to customers **Materiel, the goods and equipment for a military unit to fulfill its mission *Supply, as in confidenc ...
s,
communications Communication (from la, communicare, meaning "to share" or "to be in relation with") is usually defined as the transmission of information. The term may also refer to the message communicated through such transmissions or the field of inquir ...
, and command vulnerable. Defence in depth requires that a defender deploy their resources, such as
fortification A fortification is a military construction or building designed for the defense of territories in warfare, and is also used to establish rule in a region during peacetime. The term is derived from Latin ''fortis'' ("strong") and ''facere'' ...
s, field works and
military units Military organization or military organisation is the structuring of the armed forces of a state so as to offer such military capability as a national defense policy may require. In some countries paramilitary forces are included in a nation ...
at and well behind the front line. Although attackers may find it easier to breach the more weakly defended front line, as they advance, they continue to meet resistance. As they penetrate deeper, their flanks become vulnerable, and, should the advance stall, they risk being enveloped. The defence in depth strategy is particularly effective against attackers who are able to concentrate their forces and attack a small number of places on an extended defensive line. Defenders that can fall back to a succession of prepared positions can extract a high price from the advancing enemy while themselves avoiding the danger of being overrun or outflanked. Delaying the enemy advance mitigates the attacker's advantage of surprise and allows time to move defending units to make a defence and to prepare a counter-attack. A well-planned defence in depth strategy will deploy forces in mutually supportive positions and in appropriate roles. For example, poorly trained troops may be deployed in static defences at the front line, whereas better trained and equipped troops form a mobile reserve. Successive layers of defence may use different technologies against various targets; for example, dragon's teeth might present a challenge for
tank A tank is an armoured fighting vehicle intended as a primary offensive weapon in front-line ground combat. Tank designs are a balance of heavy firepower, strong armour, and good battlefield mobility provided by tracks and a powerful engi ...
s but is easily circumvented by
infantry Infantry is a military specialization which engages in ground combat on foot. Infantry generally consists of light infantry, mountain infantry, motorized infantry & mechanized infantry, airborne infantry, air assault infantry, and marine ...
, while another barrier of
wire entanglement In the military science of fortification, wire obstacles are defensive obstacles made from barbed wire, barbed tape or concertina wire. They are designed to disrupt, delay and generally slow down an attacking enemy. During the time that the atta ...
s has the opposite effects on the respective forces. Defence in depth may allow a defender to maximise the defensive possibilities of natural
terrain Terrain or relief (also topography, topographical relief) involves the vertical and horizontal dimensions of land surface. The term bathymetry is used to describe underwater relief, while hypsometry studies terrain relative to sea level. The ...
and other advantages. The disadvantages of defence in depth are that it may be unacceptable for a defender to plan to give ground to an attacker. This may be because vital military or economic resources are close to the front line or because yielding to an enemy is unacceptable for political or cultural reasons. In addition, the continuous
retreats The meaning of a spiritual retreat can be different for different religious communities. Spiritual retreats are an integral part of many Hindu, Jewish, Buddhist, Christian and Sufi communities. In Hinduism and Buddhism, meditative retreats are ...
that are required by defence in depth require the defender to have a high degree of
mobility Mobility may refer to: Social sciences and humanities * Economic mobility, ability of individuals or families to improve their economic status * Geographic mobility, the measure of how populations and goods move over time * Mobilities, a conte ...
in order to retreat successfully, and they assume that the defender's
morale Morale, also known as esprit de corps (), is the capacity of a group's members to maintain belief in an institution or goal, particularly in the face of opposition or hardship. Morale is often referenced by authority figures as a generic value ...
will recover from the retreat.


Examples

A possible early example of this came at the Battle of Cannae in 216 BC, when
Hannibal Hannibal (; xpu, 𐤇𐤍𐤁𐤏𐤋, ''Ḥannibaʿl''; 247 – between 183 and 181 BC) was a Carthaginian general and statesman who commanded the forces of Carthage in their battle against the Roman Republic during the Second Pun ...
employed this manoeuvre in order to encircle and destroy eight
Roman legions The Roman legion ( la, legiō, ) was the largest military unit of the Roman army, composed of 5,200 infantry and 300 equites (cavalry) in the period of the Roman Republic (509 BC–27 BC) and of 5,600 infantry and 200 auxilia in the period of t ...
, but that is disputed by some historians.Healy, Mark. ''Cannae: Hannibal Smashes Rome's Army''. Sterling Heights, Missouri: Osprey Publishing, 1994.
Edward Luttwak Edward Nicolae Luttwak (born 4 November 1942) is an American author known for his works on grand strategy, military strategy, geoeconomics, military history, and international relations. He is best known for being the author of '' Coup d'État: ...
used the term to describe his theory of the defensive strategy employed by the
Late Roman army In modern scholarship, the "late" period of the Roman army begins with the accession of the Emperor Diocletian in AD 284, and ends in 480 with the death of Julius Nepos, being roughly coterminous with the Dominate. During the period 395–476, ...
in the 3rd and 4th centuries AD. Later examples of defence in depth might be European
hill fort A hillfort is a type of earthwork used as a fortified refuge or defended settlement, located to exploit a rise in elevation for defensive advantage. They are typically European and of the Bronze Age or Iron Age. Some were used in the post-Rom ...
s and the development of
concentric castle A concentric castle is a castle with two or more concentric curtain walls, such that the outer wall is lower than the inner and can be defended from it. The layout was square (at Belvoir and Beaumaris) where the terrain permitted, or an irreg ...
s. These castles utilized many layers, including ditches, outer walls, towers, inner walls, and a keep, with some layers like the outer ditch intended to only slow attackers and reduce their coordination. In those examples, the inner layers of defence can support the outer layers with
projectile A projectile is an object that is propelled by the application of an external force and then moves freely under the influence of gravity and air resistance. Although any objects in motion through space are projectiles, they are commonly found in ...
fire and an attacker must breach each line of defence in turn with the prospect of significant losses, and the defenders have the option of falling back to fight again. On a strategic level, defence in depth was employed by the Byzantine military. In the
American Revolutionary War The American Revolutionary War (April 19, 1775 – September 3, 1783), also known as the Revolutionary War or American War of Independence, was a major war of the American Revolution. Widely considered as the war that secured the independence of t ...
's
Battle of Cowpens The Battle of Cowpens was an engagement during the American Revolutionary War fought on January 17, 1781 near the town of Cowpens, South Carolina, between U.S. forces under Brigadier General Daniel Morgan and British forces under Lieutenant Colo ...
, the American forces were positioned in three lines which soaked up the shock of the British
charge Charge or charged may refer to: Arts, entertainment, and media Films * ''Charge, Zero Emissions/Maximum Speed'', a 2011 documentary Music * ''Charge'' (David Ford album) * ''Charge'' (Machel Montano album) * '' Charge!!'', an album by The Aqu ...
and inflicted heavy casualties before the Americans were able to overrun the British who, at this point, had lost their cohesion. More recent examples of defence in depth include the multiple lines of trenches of the
First World War World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fighti ...
and the following
Turkish War of Independence The Turkish War of Independence "War of Liberation", also known figuratively as ''İstiklâl Harbi'' "Independence War" or ''Millî Mücadele'' "National Struggle" (19 May 1919 – 24 July 1923) was a series of military campaigns waged by th ...
where the Turks stopped the advance of the Greeks towards Ankara. Also plans for the defence of Britain against a potential German invasion in the
Second World War World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...
. During the
Battle of Normandy Operation Overlord was the codename for the Battle of Normandy, the Allied operation that launched the successful invasion of German-occupied Western Europe during World War II. The operation was launched on 6 June 1944 (D-Day) with the Nor ...
, Wehrmacht forces utilized the
bocage Bocage (, ) is a terrain of mixed woodland and pasture characteristic of parts of Northern France, Southern England, Ireland, the Netherlands and Northern Germany, in regions where pastoral farming is the dominant land use. ''Bocage'' may als ...
of the area, flooding of fields, and strategic placement of defences to create successive lines of defences to slow the attacking Allies in hopes that reinforcements would arrive. The Pacific Theatre also had many examples of defence in depth, with the Japanese inflicting heavy casualties on the Americans in the Battles of
Tarawa Tarawa is an atoll and the capital of the Republic of Kiribati,Kiribati
''
Saipan Saipan ( ch, Sa’ipan, cal, Seipél, formerly in es, Saipán, and in ja, 彩帆島, Saipan-tō) is the largest island of the Northern Mariana Islands, a commonwealth of the United States in the western Pacific Ocean. According to 2020 es ...
,
Peleliu Peleliu (or Beliliou) is an island in the island nation of Palau. Peleliu, along with two small islands to its northeast, forms one of the sixteen states of Palau. The island is notable as the location of the Battle of Peleliu in World War II. ...
,
Iwo Jima Iwo Jima (, also ), known in Japan as , is one of the Japanese Volcano Islands and lies south of the Bonin Islands. Together with other islands, they form the Ogasawara Archipelago. The highest point of Iwo Jima is Mount Suribachi at high. ...
, and
Okinawa is a Prefectures of Japan, prefecture of Japan. Okinawa Prefecture is the southernmost and westernmost prefecture of Japan, has a population of 1,457,162 (as of 2 February 2020) and a geographic area of 2,281 Square kilometre, km2 (880 sq mi). ...
. The best modern example of a successful defence in depth is that of the
Battle of Kursk The Battle of Kursk was a major World War II Eastern Front (World War II), Eastern Front engagement between the forces of Nazi Germany and the Soviet Union near Kursk in the southwestern USSR during late summer 1943; it ultimately became the la ...
. During the battle, the
Red Army The Workers' and Peasants' Red Army ( Russian: Рабо́че-крестья́нская Кра́сная армия),) often shortened to the Red Army, was the army and air force of the Russian Soviet Federative Socialist Republic and, afte ...
deliberately drew the Germans into an attritional battle in multiple, well-prepared defensive lines, before launching massive counter-attacks on either side of the 9th Army in the north and the 4th Panzer Army in the south. The initial German
offensive Offensive may refer to: * Offensive, the former name of the Dutch political party Socialist Alternative * Offensive (military), an attack * Offensive language ** Fighting words or insulting language, words that by their very utterance inflict inj ...
never fully penetrated the Red Army lines. By contrast, the subsequent Red Army
counter-offensive In the study of military tactics, a counter-offensive is a large-scale strategic offensive military operation, usually by forces that had successfully halted the enemy's offensive, while occupying defensive positions. The counter-offensive i ...
pushed the front line hundreds of miles westwards. Colonel Francis J. Kelly discussed the employment of defence in depth in
Army Special Forces The United States Army Special Forces (SF), colloquially known as the "Green Berets" due to their distinctive service headgear, are a special operations force of the United States Army. The Green Berets are geared towards nine doctrinal mi ...
camps during the
Vietnam War The Vietnam War (also known by other names) was a conflict in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. It was the second of the Indochina Wars and was officially fought between North Vietnam and ...
. Kelly, a former U.S. Army Special Forces commander and author of ''Vietnam Studies U.S. Army Special Forces 1961–1971'', stated in his work that the austere Special Forces fighting camps were highly functional and easily defended. While untested, this was also the planned
NATO The North Atlantic Treaty Organization (NATO, ; french: Organisation du traité de l'Atlantique nord, ), also called the North Atlantic Alliance, is an intergovernmental military alliance between 30 member states – 28 European and two Nor ...
strategy in Europe during the Cold War at the
Fulda Gap The Fulda Gap (german: Fulda-Lücke), an area between the Hesse-Thuringian border (the former Inner German border) and Frankfurt am Main, contains two corridors of lowlands through which tanks might have driven in a surprise attack by the Sovi ...
. However by the 1980s,
West Germany West Germany is the colloquial term used to indicate the Federal Republic of Germany (FRG; german: Bundesrepublik Deutschland , BRD) between its formation on 23 May 1949 and the German reunification through the accession of East Germany on 3 ...
eventually pressured NATO to abandon this orthodox doctrine as it would have entailed allowing the country to be overrun by
Warsaw Pact The Warsaw Pact (WP) or Treaty of Warsaw, formally the Treaty of Friendship, Cooperation and Mutual Assistance, was a collective defense treaty signed in Warsaw, Poland, between the Soviet Union and seven other Eastern Bloc socialist republi ...
forces before they would be finally stopped. Instead, NATO agreed to an alternative doctrine of " Forward defence," which was criticized for not only being militarily senseless that would have quickly forced NATO to resort to
tactical nuclear weapon A tactical nuclear weapon (TNW) or non-strategic nuclear weapon (NSNW) is a nuclear weapon that is designed to be used on a battlefield in military situations, mostly with friendly forces in proximity and perhaps even on contested friendly territo ...
s when Warsaw Pact broke through with their considerable conventional forces, but also that enemy considered the doctrine so provocative that striking first seemed a more viable option as a result.


Application to other fields

The concept of Defence-in-Depth (DiD) is also applied in the fields of life-threatening technologies where it is critical to avoid a disaster, or to save lives. The safety of
nuclear reactor A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from n ...
s and
radioactive waste repositories Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...
also fundamentally relies on multiple systems and redundant barriers. The principle of redundancy is essential to prevent the occurrence of dramatic failures and in case where a failure would develop to retard the progression of a potentially disastrous event and to give extra time for taking again the control of the failed system. Ultimately, if a failure cannot be avoided, DiD also contributes to mitigate the consequences and to attenuate the negative impacts of the failure. Defence-in-Depth is required to guarantee the robustness of vital systems, ''e.g.'' in nuclear technologies and for
aerospace Aerospace is a term used to collectively refer to the atmosphere and outer space. Aerospace activity is very diverse, with a multitude of commercial, industrial and military applications. Aerospace engineering consists of aeronautics and astr ...
systems where safety is critical. The DiD approach can be applied to any sensitive technology: submarines and naval systems, biotechnology, pharmaceutic industry, informatics, bank and financial systems, etc. In nature, the
immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinter ...
of most evolved organisms also appeals to multiple lines of defence in case a pathogen would defeat the first line of defence of
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
, tissues and
organs In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a ...
. The robustness of the
scientific method The scientific method is an empirical method for acquiring knowledge that has characterized the development of science since at least the 17th century (with notable practitioners in previous centuries; see the article history of scientific me ...
also relies on multiple lines of evidence and multiple lines of reasoning: strong claims require strong and multiple evidence. The
repeatability Repeatability or test–retest reliability is the closeness of the agreement between the results of successive measurements of the same measure, when carried out under the same conditions of measurement. In other words, the measurements are taken ...
and the
reproducibility Reproducibility, also known as replicability and repeatability, is a major principle underpinning the scientific method. For the findings of a study to be reproducible means that results obtained by an experiment or an observational study or in ...
of experimental and calculations results are essential to guarantee their robustness and correctness. This associated with the scientific questioning and a constant interrogative attitude is at the core of the self-correcting process guiding the science. Academics from Oxford University's
Future of Humanity Institute The Future of Humanity Institute (FHI) is an interdisciplinary research centre at the University of Oxford investigating big-picture questions about humanity and its prospects. It was founded in 2005 as part of the Faculty of Philosophy and th ...
also applied the concept of defence in depth to designing strategies for the prevention of existential catastrophes, especially those involving
human extinction Human extinction, also known as omnicide, is the hypothetical end of the human species due to either natural causes such as population decline from sub-replacement fertility, an asteroid impact, or large-scale volcanism, or to anthropogenic ...
.


See also

*
Bandwagoning Bandwagoning in international relations occurs when a state aligns with a stronger, adversarial power and concedes that the stronger adversary-turned-partner disproportionately gains in the spoils they conquer together. Bandwagoning, therefore, ...
*
Culminating point The culminating point in military strategy is the point at which a military force is no longer able to perform its operations. On the offensive, the culminating point marks the time when the attacking force can no longer continue its advance, b ...
*
Deep operation Deep operation (, ''glubokaya operatsiya''), also known as Soviet Deep Battle, was a military theory developed by the Soviet Union for its armed forces during the 1920s and 1930s. It was a tenet that emphasized destroying, suppressing or disorga ...
*
Defence in depth (non-military) A defence in depth uses multi-layered protections, similar to redundant protections, to create a reliable system despite any one layer's unreliability. Examples The term defence in depth is now used in many non-military contexts. Fire preven ...
*
Defense in depth (nuclear engineering) U.S. non-military nuclear material is regulated by the U.S. Nuclear Regulatory Commission, which uses the concept of defense in depth when protecting the health and safety of the public from the hazards associated with nuclear materials. The N ...
*
Defence-in-depth (Roman military) Defence-in-depth is the term used by American political analyst Edward Luttwak (born 1942) to describe his theory of the defensive strategy employed by the Late Roman army in the third and fourth centuries AD. Luttwak's ''Grand Strategy of the ...
*
Hedgehog defence The hedgehog defence is a military tactic in which a defending army creates mutually supporting strongpoints in a defence in depth, designed to sap the strength and break the momentum of an attacking army. The strongpoints are designed to be e ...
*
List of military tactics This article contains a list of military tactics. The meaning of the phrase is context sensitive, and has varied over time, like the difference between "strategy" and "tactics". General * Exploiting prevailing weather – the tactical use of ...
*
Loss of Strength Gradient The Loss of Strength Gradient (LSG) is a military concept devised by Kenneth E. Boulding in his 1962 book ''Conflict and Defense: A General Theory''. He argued that the amount of a nation's military power that could be brought to bear in any par ...
*
Scorched earth policy A scorched-earth policy is a military strategy that aims to destroy anything that might be useful to the enemy. Any assets that could be used by the enemy may be targeted, which usually includes obvious weapons, transport vehicles, communi ...
*
Strategic depth Strategic depth is a term in military literature that broadly refers to the distances between the front lines or battle sectors and the combatants' industrial core areas, capital cities, heartlands, and other key centers of population or milita ...


References

{{Use British English, date=October 2010 Military strategy Military doctrines Military geography de:Gefechtsart#Verzögerung