INTERIOR LINES. A term used in tactics and strategy to indicate a situation in which one commander has an advantage in being able to employ his forces against the enemy faster than the enemy can counter his moves. A commander may possess interior lines by virtue of a central position with respect to his opponent. This is so self-evident that one is led into error in assuming that there is nothing more to the concept of interior lines. But a commander may also possess interior lines by virtue of having superior lateral communications. Consider Washington's dilemma at the start of the Philadelphia campaign: he was located in New Jersey; the British were in New York City; and Burgoyne's offensive was moving south along the Lake Champlain-Hudson River line. Washington had a "central position" from which, in theory, he could move the bulk of his forces to meet Burgoyne's threat in the north or any of three threats from General Howe in New York City: up the Hudson to join Burgoyne; overland through New Jersey to Philadelphia; or by sea to the Delaware and against Philadelphia. Yet by virtue of their superior lateral communications—which in this instance were by water—the British actually had interior lines.
An understanding of interior lines and a correct use of the concept has been a hallmark of successful tacticians and strategists through the ages; the concept has been misunderstood by other military men and by most writers for the same period. The main purpose of this article is to put the reader on guard: it is beyond the scope of the present work to attempt a complete explanation of what interior lines are, but it is possible to point out what they are not.
Before leaving the subject, however, it should be noted that a commander who does not possess the advantage of interior lines at the start of a campaign may often create the situation by a "strategic penetration." The campaigns of Napoleon offer many examples.