nal 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.
Consequently the opening moves which we generally made to defend our trade
by the occupation of those lines placed us in a position to attack our
enemy's trade. The same situation arose even when our opening dispositions
were designed as defence against home invasion or against attacks upon our
colonies, for the positions our fleet had to take up to those ends always
lay on or about the terminal and focal points of trade routes. Whether our
immediate object were to bring the enemy's main fleets to action or to
exercise economic pressure, it made but little difference. If the enemy
were equally anxious to engage, it was at one of the terminal or focal
areas we were almost certain to get contact. If he wished to avoid a
decision, the best way to force him to action was to occupy his trade
routes at the same vital points.

Thus it comes about that, whereas on land the process of economic pressure,
at least in the modern conception of war, should only begin after decisive
victory, at sea it starts automatically from the first. Indeed such
pressure may be the only means of forcing the decision we seek, as will
appear more clearly when we come to deal with the other fundamental
difference between land and sea warfare.