Defence in depth

Defense in depth (also known as deep or elastic defense) is a military strategy. It is a delaying tactic intended to slow down the advance of an enemy, instead of stopping it. The tactic buys time by yielding to the enemy slowly and usually causes additional casualties. A defense in depth can 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 center. On either side were his most experienced fighters. When the Romans advanced, his center 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 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 missiles fire. The attacker has to breach each line of defense exhausting himself 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 and the British Armies until July 1918. The arrival of the US Army, which joined the French and the British, ended that German tactic.
Strategy.
A properly-planned defense in depth can be used to reduce or eliminate any advantage an attacking force might have,such as superior numbers. The defender places the object of his attack behind several layers of defense. The defender then lets the attacker wear down his forces while they slowly give ground and then he moves back to the next layer of defense.
Carl von Clausewitz, Sun Tzu, and the martial art of Budō all agreed that the preferred form of war was defense. Clausewitz stated that defense provided the defenders with additional . A defense in depth can prevent an enemy from surrounding a position and provides an excellent opportunity to counterattack. A defense in space is not being there when an enemy attacks. A defense in time means slowing down or blocking an enemy when it attacks.
Delaying action.
A similar tactic is called a delaying action. The object is for a smaller force to harass a larger force and to delay its advance while inflicting as much damage as possible to the larger force without directly engaging it. This allows the defending army's main force to disengage an enemy while maintaining 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.