Not to be confused with Defense in depth (computing).
A medieval castle on a hill with multiple walls and obstacles is an example of a defense in depth

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**Defense in depth** (also known as **deep** or **elastic defense**) is a military strategy. The delaying tactic is intended not to stop the advance of an enemy but to slow it down. A defense in deep buys time by slowly yielding to the enemy and usually causes additional casualties. That may slow down an advancing army and cause it to lose momentum.

Examples
--------

Hannibal used the tactic at the Battle of Cannae in 216 BC. Facing a much larger Roman army, he placed his less experienced soldiers in the centre and his most experienced fighters on both sides. When the Romans advanced, his centre gradually moved back, and the troops on the wings began to surround the Romans. That was the largest slaughter of Roman soldiers in the history of the Roman Republic.

The classic example is medieval hill forts and castles with rings of defenses (usually walls). The inner circles of defenders support the outer circles with missile fire. The attackers must breach all of the lines of defense and exhaust themselves in the process.

The German Army started to use the tactic in 1917, during World War I. and continued the tactic with a great effect against both the French Army and the British Army until July 1918. The arrival of the US Army, which joined the French and the British, ended the effectiveness of the tactic.

Strategy
--------

A properly-planned defense in depth may reduce or eliminate any advantage that attacking forces might havesuch as superior numbers. The defenders place the the attackers behind several layers of defense. The defenders then let the attackers wear down their forces, and the defenders slowly give ground and move back to the next layer of defense.

Carl von Clausewitz, Sun Tzu, and the martial art of Budō consider the preferred form of war to be defense. Clausewitz stated that provides the defenders with additional opportunities. A defence in depth may prevent an enemy from surrounding a position and provides an excellent opportunity to counterattack.

A **defense in space** is being absent when an enemy attacks. A **defense in time** is slowing down or blocking an enemy that attacks.

Delaying action
---------------

A similar tactic is called a **delaying action**. when a smaller force to harasses a larger force to delay its advance and to inflict as much damage as possible to the larger force without directly engaging it. That allows the defending army's main force to disengage an enemy and to maintain good order. The main force is given the time necessary to set up a new defensive position. The small force protecting the larger force is called a rearguard.

A famous rearguard example was given in the *Song of Roland*. The nephew of Charlemagne, Roland, commanded the rearguard at the Battle of Roncevaux Pass. He and his men protected the rear of the Frankish army while it retreated to France. In his delaying action, Roland and all of his men were killed in an ambush.

Other websites
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* Defence-in-Depth, King's College, London