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Use of interior lines is a strategy of warfare based on the fact that lines of movement and communication within an enclosed area are shorter than those on the outside. As the area held by a defensive force shrinks, the advantages increase.
Using the strategy of interior lines, a partially surrounded or more centrally disposed force can more easily resupply and redeploy its units, and thus more easily mount a series of quick attacks at multiple locations.
In the context of battlefield tactics, it allows more rapid concentration of resources (firepower and manpower) and so affords greater tactical flexibility. The resources are ideally brought to bear at a point where the adversary is not able to quickly respond, because of their longer external lines:
- At the Battle of Dyrrhachium (48 BC), the numerically superior Optimates, led by Gnaeus Pompeius Magnus, defeated the more battle-hardened Populares, led by Gaius Julius Caesar, when Caesar attempted to encircle them.
- During the Battles of Lexington and Concord, Brigadier General Hugh Percy utilized interior lines during the British retreat, as his men were often surrounded by militia.
- At the Battle of Jena-Auerstedt, Louis Nicolas Davout employed interior lines to defeat the main Prussian army.
- At the Battle of Wagram, the French under Napoleon I defeated the Austrians by acquiring interior lines and achieving local numerical superiority.
- George Gordon Meade used it against Robert E. Lee at the Battle of Gettysburg in 1863.
As a strategy, it is commonly employed to cut armies off from reinforcements supplies, or prevent allies from uniting their forces. That usually allows for an adversary that may not be numerically superior to gain a numerical superiority over an adversary in a given locality, which increases the chances of overpowering an enemy and defeating it. By extension, in overpowering an enemy, an adversary hopes to demoralize it to a sufficient degree to bring it to political terms.
- At the Battle of Montenotte, the First French Republic under Napoleon defeated the Austrians and destroyed an entire corps, thereby destroying the link between the Austrians and their allies Kingdom of Sardinia. As a result of the victory, the Sardinians were separated from Austria and were unable to defeat the French or rejoin the Austrians. They eventually sued for peace.
- Frederick the Great's operational strategy, in his prosecution of the Seven Years' War against the separate armies of the French, the Russians and the Austrians, can be considered an example of the advantage of interior lines in warfare.
- It also gave the Reds a distinct advantage over the Whites in the Russian Civil War.
- General Robert E. Lee used the strategy during the Battle of Antietam during the American Civil War in 1862.
- Though the Pusan Perimeter (Korea, 1950) was not an intentional strategy, the concentration of UN forces within the Perimeter allowed quick movement of supplies and reinforcements via internal lines.