Defence in depth
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Defence in depth (also known as deep or elastic defence) is a military strategy; it 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 a period of 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 warfare or drive the attacker back to its original starting position.
The idea of defence in depth is now widely used to describe multi-layered or redundant protections for non-military situations, both tactical and strategic.
Military defence in depth 
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 his resources, such as fortifications, field works and military units at and well behind the front line. Although an attacker may find it easier to breach the more weakly defended front line, as he advances he continues to meet resistance. As he penetrates deeper, the attacker's flanks become vulnerable, and, should the advance stall, the attacker risks being enveloped.
The defence in depth strategy is particularly effective against an attacker who is able to concentrate his forces and attack a small number of places on an extended defensive line.
Defenders who 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 required by defence in depth require the defender to have a high degree of mobility in order to retreat successfully and the morale to recover from the retreat.
An early example of this came at the Battle of Cannae in 216 B.C. when Hannibal employed this manoeuvre in order to encircle and destroy 10 Roman Legions all at once, resulting in the largest single slaughter of Roman troops in the history of the republic.
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. In these examples, the inner layers of defence can support the outer layers with missile fire and an attacker must breach each line of defence in turn with the prospect of significant losses, whereas the defenders have the option of falling back to fight again. In the American Revolutionary War 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, plans for the defence of Britain under threat of German invasion. During the Battle of Normandy, Wehrmacht forces utilized the bocage of the area to create successive lines of defences to slow the attacking Allies in hopes that reinforcements would arrive. The Pacific Front also had many examples of Defence in Depth where the Japanese inflicted heavy casualties on the Americans in battles such as Tarawa, Saipan, Peleliu, Iwo Jima, and Okinawa. Colonel Francis J. Kelly discussed the employment of the defence in depth principle in Army Special Forces camps during American involvement in Vietnam. 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.
Non-military examples 
The term defence in depth is now used in many non-military contexts.
Fire prevention 
A defence in depth strategy to fire prevention does not focus all the resources only on the prevention of a fire; instead, it also requires the deployment of fire alarms, extinguishers, evacuation plans, mobile rescue and fire-fighting equipment and even nation-wide plans for deploying massive resources to a major blaze.
Defence in depth may mean engineering which emphasizes redundancy - a system that keeps working when a component fails - over attempts to design components that will not fail in the first place. For example, an aircraft with four engines will be less likely to suffer total engine failure than a single-engined aircraft no matter how much effort goes into making the single engine reliable.
Nuclear engineering 
In nuclear engineering and nuclear safety, defence in depth denotes the practice of having multiple, redundant, and independent layers of safety systems for the single, critical point of failure: the reactor core. The aim is to reduce the risk that a single failure of a critical system could cause a core meltdown or a catastrophic failure of reactor containment. With the 2011 Japanese Fukushima nuclear disaster, all of those lines of defence collapsed. The Fukushima disaster has thrown into question ways of thinking about nuclear safety.
Information security 
Likewise, in information security / Information Assurance defence in depth represents the use of multiple computer security techniques to help mitigate the risk of one component of the defence being compromised or circumvented. An example could be anti-virus software installed on individual workstations when there is already virus protection on the firewalls and servers within the same environment. Different security products from multiple vendors may be deployed to defend different potential vectors within the network, helping prevent a shortfall in any one defence leading to a wider failure; also known as a "layered approach".
See also 
- Culminating point
- Hedgehog defence
- Loss of Strength Gradient
- Scorched earth policy
- Soviet deep battle
- Strategic depth
- Martin Fackler (1 June 2011). "Report Finds Japan Underestimated Tsunami Danger". New York Times.
- International Nuclear Energy Agency (1996). "Defence in depth in nuclear safety (INSAG-10)". ISBN 92-0-103295-1.
- Declan Butler (11 January 2012). "France 'imagines the unimaginable'". Nature.