Defence in depth

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 to slow down the advance of an enemy, instead of stopping it. That buys time by yielding to the enemy slowly and usually causes additional casualties. A defense in depth 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. On both sides were his most experienced fighters. When the Romans advanced, his centre gradually moved back while 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 defences (usually walls). The inner circles of defenders support the outer circles with missile fire. The attackers must breach each line of defence and exhaust themselves in the process.

The German Army used the tactic in 1917, during World War I. The Germans used it to 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 defence 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 defence. The defenders then let the attackers wear down their forces while the defenders slowly give ground and move back to the next layer of defence.

Carl von Clausewitz, Sun Tzu, and the martial art of Budō agree that the preferred form of war is defence. Clausewitz states 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. The object is for a smaller force to harass a larger force to delay its advance, and to inflictil 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 back to France. In his delaying action, Roland and all of his men were killed in an ambush.

Other websites

• Defence-in-Depth, King's College, London