What does 'defense in depth' aim to accomplish?

'Defense in depth' is a military strategy designed to create multiple layers or positions of defense to absorb and weaken enemy attacks progressively. This approach aims to prevent a single catastrophic failure by spreading defense across various positions or units, which can delay and disrupt the enemy's advance. As the enemy engages with each defensive layer, they not only face increased resistance but also become exposed to counteractions from other units, leading to a depletion of their resources and effectiveness.

This method allows defending forces to manage and respond to threats dynamically. Rather than relying on a single line of defense that, if breached, could result in rapid collapse, 'defense in depth' provides the opportunity for a structured withdrawal to subsequent defensive positions if necessary, maintaining cohesion and the ability to launch counteroffensives.

In contrast, establishing a single line of defense, enabling easy commitment of reserves, or concentrating all units at one point would undermine the key strengths of a layered defense, such as flexibility, resilience, and the ability to engage the enemy effectively over an extended front. These tactics do not capitalize on the primary benefits of dispersing forces to absorb the impact of an attack effectively.