Defence in depth
Defence in depth is a military strategy
Military strategy
Military strategy is a set of ideas implemented by military organizations to pursue desired strategic goals. Derived from the Greek strategos, strategy when it appeared in use during the 18th century, was seen in its narrow sense as the "art of the general", 'the art of arrangement' of troops...

; 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
Logistics is the management of the flow of goods between the point of origin and the point of destination in order to meet the requirements of customers or corporations. Logistics involves the integration of information, transportation, inventory, warehousing, material handling, and packaging, and...

 or spread out a numerically superior attacking force. Once an attacker has lost momentum or is forced to spread out to pacify
Peace is a state of harmony characterized by the lack of violent conflict. Commonly understood as the absence of hostility, peace also suggests the existence of healthy or newly healed interpersonal or international relationships, prosperity in matters of social or economic welfare, the...

 a large area, defensive counter-attack
Counter-Attack is a 1945 war film starring Paul Muni and Marguerite Chapman as two Russians trapped in a collapsed building with seven enemy German soldiers during World War II...

s can be mounted on the attacker's weak points with the goal being to cause attrition warfare
Attrition warfare
Attrition warfare is a military strategy in which a belligerent side attempts to win a war by wearing down its enemy to the point of collapse through continuous losses in personnel and matériel....

 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
Front line
A front line is the farthest-most forward position of an armed force's personnel and equipment - generally in respect of maritime or land forces. Forward Line of Own Troops , or Forward Edge of Battle Area are technical terms used by all branches of the armed services...

 which, if breached by an attacker, would leave the remaining defenders in danger of being outflanked
Flanking maneuver
In military tactics, a flanking maneuver, also called a flank attack, is an attack on the sides of an opposing force. If a flanking maneuver succeeds, the opposing force would be surrounded from two or more directions, which significantly reduces the maneuverability of the outflanked force and its...

 and surrounded and would leave supply lines communications and command vulnerable.

Defence in depth requires that a defender deploy his resources, such as fortification
Fortifications are military constructions and buildings designed for defence in warfare and military bases. Humans have constructed defensive works for many thousands of years, in a variety of increasingly complex designs...

s, 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
Counter-Attack is a 1945 war film starring Paul Muni and Marguerite Chapman as two Russians trapped in a collapsed building with seven enemy German soldiers during World War II...


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
Cannon fodder
Cannon fodder is an informal, derogatory term for military personnel who are regarded or treated as expendable in the face of enemy fire. The term is generally used in situations where soldiers are forced to deliberately fight against hopeless odds in an effort to achieve a strategic goal...

 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 or tactics; for example, a row of dragon's teeth
Dragon's teeth (fortification)
Dragon's teeth are square-pyramidal fortifications of reinforced concrete first used during the Second World War to impede the movement of tanks and mechanised infantry...

 may be a problem for tanks but no barrier to infantry, while another barrier of wire entanglements has the opposite effects. 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
Morale, also known as esprit de corps when discussing the morale of a group, is an intangible term used to describe the capacity of people to maintain belief in an institution or a goal, or even in oneself and others...

 to recover from the retreat.

An early example of this came at the Battle of Cannae
Battle of Cannae
The Battle of Cannae was a major battle of the Second Punic War, which took place on August 2, 216 BC near the town of Cannae in Apulia in southeast Italy. The army of Carthage under Hannibal decisively defeated a numerically superior army of the Roman Republic under command of the consuls Lucius...

 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.

Later examples of defence in depth might be European hill fort
Hill fort
A hill fort is a type of earthworks 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 and Iron Ages. Some were used in the post-Roman period...

s and the development of concentric castle
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 word concentric does not imply that these castles were circular; in fact if taken too literally the term "concentric" is quite misleading...

s. 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
American Revolutionary War
The American Revolutionary War , the American War of Independence, or simply the Revolutionary War, began as a war between the Kingdom of Great Britain and thirteen British colonies in North America, and ended in a global war between several European great powers.The war was the result of the...

 Battle of Cowpens
Battle of Cowpens
The Battle of Cowpens was a decisive victory by Patriot Revolutionary forces under Brigadier General Daniel Morgan, in the Southern campaign of the American Revolutionary War...

, 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
British anti-invasion preparations of World War II
British anti-invasion preparations of the Second World War entailed a large-scale division of military and civilian mobilisation in response to the threat of invasion by German armed forces in 1940 and 1941. The British army needed to recover from the defeat of the British Expeditionary Force in...

. During the Battle of Normandy
Operation Overlord
Operation Overlord was the code name for the Battle of Normandy, the operation that launched the invasion of German-occupied western Europe during World War II by Allied forces. The operation commenced on 6 June 1944 with the Normandy landings...

, Wehrmacht
The Wehrmacht – from , to defend and , the might/power) were the unified armed forces of Nazi Germany from 1935 to 1945. It consisted of the Heer , the Kriegsmarine and the Luftwaffe .-Origin and use of the term:...

 forces utilized the bocage
Bocage is a Norman word which has entered both the French and English languages. It may refer to a small forest, a decorative element of leaves, a terrain of mixed woodland and pasture, or a type of rubble-work, comparable with the English use of 'rustic' in relation to garden...

 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
Battle of Tarawa
The Battle of Tarawa, code named Operation Galvanic, was a battle in the Pacific Theater of World War II, largely fought from November 20 to November 23, 1943. It was the first American offensive in the critical central Pacific region....

, Saipan
Battle of Saipan
The Battle of Saipan was a battle of the Pacific campaign of World War II, fought on the island of Saipan in the Mariana Islands from 15 June-9 July 1944. The Allied invasion fleet embarking the expeditionary forces left Pearl Harbor on 5 June 1944, the day before Operation Overlord in Europe was...

, Peleliu
Battle of Peleliu
The Battle of Peleliu, codenamed Operation Stalemate II, was fought between the United States and the Empire of Japan in the Pacific Theater of World War II, from September–November 1944 on the island of Peleliu, present-day Palau. U.S...

, Iwo Jima
Battle of Iwo Jima
The Battle of Iwo Jima , or Operation Detachment, was a major battle in which the United States fought for and captured the island of Iwo Jima from the Empire of Japan. The U.S...

, and Okinawa
Battle of Okinawa
The Battle of Okinawa, codenamed Operation Iceberg, was fought on the Ryukyu Islands of Okinawa and was the largest amphibious assault in the Pacific War of World War II. The 82-day-long battle lasted from early April until mid-June 1945...

. Colonel Francis J. Kelly discussed the employment of the defence in depth principal 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 defence in depth

The term defence in depth is now used in many non-military contexts.

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
Redundancy (engineering)
In engineering, redundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system, usually in the case of a backup or fail-safe....

 - 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
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

 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.

In nuclear engineering
Nuclear engineering
Nuclear engineering is the branch of engineering concerned with the application of the breakdown as well as the fusion of atomic nuclei and/or the application of other sub-atomic physics, based on the principles of nuclear physics...

and nuclear safety
Nuclear safety
Nuclear safety covers the actions taken to prevent nuclear and radiation accidents or to limit their consequences. This covers nuclear power plants as well as all other nuclear facilities, the transportation of nuclear materials, and the use and storage of nuclear materials for medical, power,...

, 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. This helps to reduce the risk that a single failure of a critical system could cause a core meltdown or a catastrophic failure of reactor containment.

Likewise, in information security
Information security
Information security means protecting information and information systems from unauthorized access, use, disclosure, disruption, modification, perusal, inspection, recording or destruction....

or Information Assurance
Information Assurance
Information assurance is the practice of managing risks related to the use, processing, storage, and transmission of information or data and the systems and processes used for those purposes...

defence in depth represents the use of multiple computer security
Computer security
Computer security is a branch of computer technology known as information security as applied to computers and networks. The objective of computer security includes protection of information and property from theft, corruption, or natural disaster, while allowing the information and property to...

 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 server
Server (computing)
In the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...

s within the same environment. Different security products from multiple vendors may be deployed to defend different potential vectors within the network
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....

, helping prevent a shortfall in any one defence leading to a wider failure; also known as a "layered approach".

See also

  • Hedgehog defence
    Hedgehog defence
    In warfare, the hedgehog defence is a military tactic for defending against a mobile armoured attack, or blitzkrieg. The defenders deploy in depth in heavily fortified positions suitable for all-around defence. The attackers can penetrate between these "hedgehogs", but each position continues to...

  • Soviet deep battle
  • Strategic depth
    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 military production...

  • Scorched earth policy
  • Culminating point
    Culminating point
    The culminating point in military strategy is the point at which a military force no longer is able to perform its operations.On the offensive, the culminating point marks the time when the attacking force can no longer continue its advance, because of supply problems, the opposing force, or the...

  • Loss of Strength Gradient
    Loss of Strength Gradient
    The Loss of Strength Gradient was devised by Kenneth Boulding in 1962. He argued that the amount of a nation’s military power that could be brought to bear in any part of the world depended on geographic distance. The Loss of Strength Gradient demonstrated, in graphical form, that the further away...

  • Strategic defence
    Strategic defence
    A Strategic defence is a type of military planning doctrine and a set of combat activities used for the purpose of deterring, resisting and repelling a strategic offensive, conducted as either a territorial or airspace invasion, or a naval offensive to interrupt shipping lane traffic as a form of...

  • Defence-in-depth (Roman military)
    Defence-in-depth (Roman military)
    Defence-in-depth is the term used by American political analyst Edward Luttwak to describe his theory of the defensive strategy employed by the Late Roman army in the 3rd and 4th centuries AD....

  • Defence in depth (computing)
    Defense in Depth (computing)
    Defense in depth is an information assurance concept in which multiple layers of security controls are placed throughout an information technology system...

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