Lines of communication.
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.