Defence in depth 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 - attack s 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 line s , communications , and command vulnerable . Defence in depth requires that a defender deploy his resources , such as fortification 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 . 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 entanglement s 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 . Examples . 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 fort s and the development of concentric castle 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 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 . Defense - in - depth is incorporated into fire protection regulations for nuclear power plants . It requires preventing fires , detecting and extinguishing fires that do occur , and ensuring the capability to safely shutdown . Engineering . 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 firewall s and servers within the same environment . Different security products from multiple vendors may be deployed to defend different potential vector s within the network , helping prevent a shortfall in any one defence leading to a wider failure ; also known as a " layered approach " .