The primary method, then, in which we use victory or preponderance at sea and bring it to bear on the enemy’s population to secure peace, is by the capture or destruction of the enemy’s property, whether public or private.  But in comparing the process with the analogous occupation of territory and the levying of contributions and requisitions we have to observe a marked difference.  Both processes are what may be called economic pressure.  But ashore the economic pressure can only be exerted as the consequence of victory or acquired domination by military success.  At sea the process begins at once.  Indeed, more often than not, the first act of hostility in maritime wars has been the capture of private property at sea.  In a sense this is also true ashore.  The first step of an invader after crossing the frontier will be to control to a less or greater extent such private property as he is able to use for his purposes.  But such interference with private property is essentially a military act, and does not belong to the secondary phase of economic pressure.  At sea it does, and the reason why this should be so lies in certain fundamental differences between land and sea warfare which are implicit in the communication theory of naval war.

To elucidate the point, it must be repeated that maritime communications, which are the root of the idea of command of the sea, are not analogous to military communications in the ordinary use of the term.  Military communications refer solely to the army’s lines of supply and retreat.  Maritime communications have a wider meaning.  Though in effect embracing the lines of fleet supply, they correspond in strategical values not to military lines of supply, but to those internal lines of communication by which the flow of national life is maintained ashore.  Consequently maritime communications are on a wholly different footing from land communications.  At sea the communications are, for the most part, common to both belligerents, whereas ashore each possesses his own in his own territory.  The strategical effect is of far-reaching importance, for it means that at sea strategical offence and defence tend to merge in a way that is unknown ashore.  Since maritime communications are common, we as a rule cannot attack those of the enemy without defending our own.  In military operations the converse is the rule.  Normally, an attack on our enemy’s communications tends to expose their own.

The theory of common communications will become clear by taking an example.  In our wars with France our communications with the Mediterranean, India, and America ran down from the Channel mouth past Finisterre and St. Vincent; and those of France, at least from her Atlantic ports, were identical for almost their entire distance.  In our wars with the Dutch the identity was even closer.  Even in the case of Spain, her great trade routes followed the same lines as our own for the greater part of their extent.