Interior lines (as opposed to exterior lines) is a military term, derived from the generic term line of operation or line of movement. The term "interior lines" is commonly used to illustrate, describe, and analyze the various possible routes (lines) of logistics, supply, recon, approach, attack, evasion, maneuver, or retreat of armed forces. Interior line strategies are based on the fact that lines of movement and communication within an enclosed area are shorter and safer than those on the outside. As the area held by a defensive force shrinks, the interior line advantage held by the defensive force increases. Skillful and comprehensive application of interior line tactics can, for a partially surrounded combat force, provide vital breathing space, greatly reduce time, effort, security, and secrecy of resupplies and redeployment, and decrease the number of casualties. These effects may have a great impact on morale and eventually prove decisive, as according to Napoleon Bonaparte: "in war morale forces are to physical three to one".

Tactic

In the context of battlefield tactics, interior lines allow for more rapid concentration of resources (firepower and manpower) and afford greater tactical flexibility. Resources are ideally brought to bear at a point where the adversary is not able to quickly respond, because of their longer external lines. Examples include:

Strategy

As a strategy, interior lines are commonly employed to cut armies off from reinforcements and supplies, or prevent allies from uniting their forces. Interior lines often allow for a numerically inferior force to gain a numerical superiority over an adversary in a given locality, which increases the chances of overpowering an enemy and defeating it in detail. In overpowering an enemy locally, an army hopes to demoralize the enemy sufficiently to bring it to political terms. Some examples include: