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.
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.
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.
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.
[1] 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.
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.
Colonel Francis J. Kelly discussed the employment of defence in depth in Army Special Forces camps during the Vietnam War.
[citation needed] 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.