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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.
