Defence in depth (also known as deep defence or elastic defence) is a military strategy that seeks to delay rather than prevent the advance of an attacker, buying time and causing additional casualties 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 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-attacks 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, which, if breached by an attacker, would leave the remaining defenders in danger of being outflanked and surrounded and would leave supply lines, communications, and command vulnerable.

Defence in depth requires that a defender deploy their resources, such as fortifications, field works and military units 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 tanks but is easily circumvented by infantry, while another barrier of wire entanglements has the opposite effects on the respective forces. Defence in depth may allow a defender to maximise the defensive possibilities of natural terrain 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 that are required by defence in depth require the defender to have a high degree of mobility in order to retreat successfully, and they assume that the defender's morale will recover from the retreat.

Examples
A possible early example of this came at the Battle of Cannae in 216 BC, when Hannibal employed this manoeuvre in order to encircle and destroy eight Roman legions, but that is disputed by some historians.

Edward Luttwak used the term to describe his theory of the defensive strategy employed by the Late Roman army in the 3rd and 4th centuries AD.

Later examples of defence in depth might be European hill forts and the development of concentric castles. 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 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's Battle of Cowpens, the American forces were positioned in three lines which soaked up the shock of the British charge 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 and the following Turkish War of Independence 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. During the Battle of Normandy, Wehrmacht forces utilized the bocage 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, Saipan, Peleliu, Iwo Jima, and Okinawa.

The best modern example of a successful defence in depth is that of the Battle of Kursk. During the battle, the Red Army 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 never fully penetrated the Red Army lines. By contrast, the subsequent Red Army counter-offensive pushed the front line hundreds of miles westwards.

Colonel Francis J. Kelly discussed the employment of defence in depth in Army Special Forces camps during the Vietnam War. 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 strategy in Europe during the Cold War at the Fulda Gap. However, by the 1980s, West Germany eventually pressured NATO to abandon this orthodox doctrine as it would have entailed allowing the country to be overrun by Warsaw Pact 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 weapons when Warsaw Pact broke through with their considerable conventional forces, but also that enemy considered the doctrine so provocative and potentially aggressive that striking first seemed a 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 reactors and radioactive waste repositories 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 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 of most evolved organisms also appeals to multiple lines of defence in case a pathogen would defeat the first line of defence of cells, tissues and organs.

The robustness of the  scientific method also relies on multiple lines of evidence and multiple lines of reasoning: strong claims require strong and multiple evidence. The repeatability and the reproducibility 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 also applied the concept of defence in depth to designing strategies for the prevention of existential catastrophes, especially those involving human extinction.

See also

 Bandwagoning
 Culminating point
 Deep operation
 Defence in depth (non-military)
 Defense in depth (nuclear engineering)
 Defence-in-depth (Roman military)
 Hedgehog defence
 List of military tactics
 Loss of Strength Gradient
 Scorched earth policy
 Strategic depth

References

Military strategy
Military doctrines
Military geography

de:Gefechtsart#Verzögerung
종심방어(縱深防禦)는 병력을 길게 늘어선 거점들에 분산 배치하여 형성된 아군의 세력권을 이용한 방어이다. 가장 대표적인 종심방어전술로  얇은 방어선을 여러 겹으로 깔아서 적의 공격을 둔화시키고 소모시키는 과정을 통해 전선을 유지하는 전술을 꼽을 수 있다. 기본적으로 적의 공격을 방어하는 방어거점을 전선 최전방으로부터 차례차례 배치하고 공격해오는 적을 차례차례 소모시킨다. 방어부대는 효과적인 지연전을 펼친 후 전선 최후방으로 빠져 재편성한 후 다시 전선으로 배치된다. 또한 후방에는 기동 예비대를 두어 적의 공격능력이 소모된 시점에서 역습을 가해 적의 공격부대를 막고 오히려 약화된 적 전선에 대해 반격으로 나설 수 있도록 한다.

종심방어전술의 발전 양상 
1차 세계 대전 당시 철조망 - 참호 - 기관총 진지 - 포병으로 구성된 종심방어진은 기동력이 없는 보병의 공격에 대해 거의 완벽한 방어를 해 내게 된다. 이후 기갑부대에 의한 전격전이 도입되면서 종심방어진은 일렬로 죽 늘어선 참호선 대신 거점 (소도시, 교통선의 교차점, 철도 집결지 등)이나 병목 (다리가 놓인 곳, 산악지대의 도로 등)에 방어역량을 집중시켜 적의 공격력을 소모시키는 지연전 형태로 바뀐다. 또한 우세한 선봉 기갑부대나 후속으로 투입되는 OMG 등에 대비하기 위해 방어선 후방에 기동화된 예비부대를 두어 적의 주공이 약화되는 시점 또는 후속부대의 전선 초월 시점에 역습을 가하는 형태로 발전한다. 또한 전투종심을 확대하기 위하여 기동화된 강습부대를 운용하여 적지 후방에 공격거점을 확보하는 식으로 전선을 오히려 적 쪽으로 밀어버리는 전술도 출현한다.

실제 전투에서의 종심방어전술

기타 
기원전 216년 칸나이 전투에서 초기 예가 나왔는데, 한니발은 로마 군단 8 명을 포위하여 파괴하기 위해이 작전을 사용했지만 일부 역사가들에 의해 논쟁이 벌어지고 있다. [1]

에드워드 루트 왁 (Edward Luttwak)은이 용어를 사용하여 서기 3 세기와 4 세기에 로마 후기 군대가 사용한 방어 전략 이론을 설명.

종심방어의 예는 유럽의 언덕 요새와 동심 성을 들 수 있다. 이 예에서 내부 방어 계층은 화력지원으로 외부 계층을 지원할 수 있으며 공격자는 상당한 손실이 발생할 것으로 예상되는 각 방어선을 차례로 공략해야하며 방어자는 전선회복 전투를 벌일 수 있다. 전략적 차원에서 비잔틴 군은 종심방어를 채택했다.

미국 독립 전쟁의 Cowpens 전투에서 미군은 영국군의 공격을 방어하고 미국인이 영국을 압도하기 전에 세 개의 전선을 벌여 많은 사상자를 냈다.

1차 세계대전, 2차 세계대전 시기의 양상 
태평양 전투. (태평양 전쟁 또한 종심방어의 사례가 많았으며, 일본은 Tarawa, Saipan, Peleliu, Iwo Jima 및 Okinawa의 전투에서 해당 전술로 미군에게 큰 피해를 입혔다.)

르제프 전투

쿠르스크 전투 (소련군은 의도적으로 여러 개의 잘 방어 된 방어선에서 독일군의 공세를 유도, 북쪽의 9 군과 남쪽의 제 4 기갑군의 양측에 대규모 반격을 시작했다. 최초의 독일 공세는 결코 소련진영에 완전히 침투하지 않았다. 대조적으로, 후속하는 붉은 군대의 반격은 최전방을 서쪽으로 수백 마일로 추격했다.)

노르망디 전투 (노르망디 전투중 베르 마 흐트 군은이 지역의 보전, 야전 범람 및 방어의 전략적 배치를 이용하여 공격 군을 늦추기 위해 연속적인 방어선을 만들었다.)

현대전에서의 양상 
베트남 전쟁 (Francis J. Kelly 대령은 베트남 전쟁 동안 육군 특수 부대 캠프에서 종심방어 도입에 대해 논의했다. 전 미 육군 특수 부대 사령관이자 베트남 연구 미 육군 특수 부대 1961-1971의 저자 인 켈리는 자신의 연구에서 엄격한 특수 부대 종심방어진이 매우 기능적이고 쉽게 방어되었다고 밝혔다.)

90년대 이후 한국군의 도로견부위주 종심방어 (기존의 고지나 진지 방어에서 벗어나, 주요 도로의 거점마다 종심방어진을 설치하여, 북한군의 차량 침투를 방지하겠다는 발상. 북한 또한 차량화 기동부대를 통한 속도있는 침투를 계획할 것이라는 생각에서 비롯되었다.)

사이버전에서의 종심방어전술(레이어드 시큐리티) (하나의 보안체제가 불완전하다는 발상에서 여러 개의 보안체제로 사이버 보안을 실현한다는 구상.)

타 용어와의 차이 
종심방어는 종심전투의 하위 항목이다. 즉, 종심전투의 하위 항목에는 종심방어와 종심공격이 있는데, 종심공격은 적의 종심으로 침투한다는 개념이고, 종심방어는 아군 세력권의 종심을 이용하여 적의 공격을 방어한다는 개념이다. 종심에서 일어나는 모든 전투행위/작전을 아우르는 말이 종심전투이다.

종심공격에 대한 세부내용은 다음 문서를 참조하십시오: 종심 작전 이론

출처 
 https://www.boannews.com/media/view.asp?idx=51200
 https://web.archive.org/web/20171004160526/http://m.chosun.com/svc/article.html?sname=news#Redyho
 https://shindonga.donga.com/3/all/13/102423/1

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