An _objective_ is always subordinate to some _object_. It is a step to the attainment of that object.
A line of operation is "the area of land or sea through which we operate from our base or starting point to reach our objectives."
Lines of operation may be _exterior_ or _interior_. We are said to hold the _interior_ lines when we hold such a position, in regard to a theatre of operations, that we can reach its chief objective points, or forces, more quickly than the enemy can move to their defence or assistance. Such a position is called an _interior position_. "Exterior Lines" and "Exterior Positions" are the converse of these.
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 usually lines of supply reversed, _i.e._, leading back to the base.
For naval purposes these three ideas are best described by the term "lines of passage and communication," which were in use at the end of the 18th century, and 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.
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 have greatly increased in importance strategically with the increased dependence of modern fleets on a regular supply of coals, stores, 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.
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, as will appear from a consideration of Maritime Communications, and the extent to which they are the main preoccupation of naval operations; that is to say, all problems of Naval Strategy can be reduced to terms of "passage and communication," and this is probably the best method of solving them.