This expression is used of three different things:--
(1) _Lines of supply_, running from the base of operations to the point which the operating force has reached.
(2) _Lines of lateral communication_ by which several forces engaged in one theatre of operations can communicate with each other and move to each other's support.
(3) _Lines of retreat_, which are lines of supply reversed, _i.e._, leading back to the base.
These three ideas are best described by the term "lines of passage and communication," which we had in use at the end of the eighteenth century.
Ashore, lines of passage and communication are roads, railways, waterways, &c.
At sea, they may be regarded as those waters over which passes the normal course of vessels proceeding from the base to the objective or the force to be supplied.
In Land Strategy the great majority of problems are problems of communication. Maritime Strategy has never been regarded as hinging on communications, but probably it does so even more than Land Strategy, as will appear from a consideration of maritime communications, and the extent to which they are the main preoccupation of Naval operations.
The various kinds of Maritime Communications for or against which a fleet may have to operate are:--
(1) Its own communications, or those of its adversary (which correspond to the communications of armies operating ashore). These tend to increase in importance strategically with the increasing hunger of modern fleets (for coal, ammunition, &c).
(2) The communications of an army operating from an advanced oversea base, that is communication between the advanced and the main base.
(3) Trade Routes, that is the communications upon which depend the national resources and the supply of the main bases, as well as the "lateral" or connecting communications between various parts of belligerents' possessions.
N.B.--Such "lines of passage and communication" are the preoccupation of Naval Strategy; that is to say, problems of Naval Strategy can be reduced to terms of "passage and communication" and this is probably the best method of solving them.
NAVAL STRATEGY CONSIDERED AS A QUESTION OF PASSAGE AND COMMUNICATION
By "Naval Strategy" we mean the art of conducting the operations of the Fleet. Such operations must always have for their object "passage and communication"; that is, the Fleet is mainly occupied in guarding our own communications and seizing those of the enemy.