This teaches us the necessity to the British Empire of controlling our maritime communications, and equally teaches those who may one day be our enemies the advisability of preventing us from doing so.  The lesson in either case is driven farther home by other considerations connected with communications.  In war a belligerent has two tasks before him.  He has to defend himself and hurt his enemy.  The more he hurts his enemy, the less is he likely to be hurt himself.  This defines the great principle of offensive defence.  To act in accordance with this principle, a belligerent should try, as the saying goes, to carry the war into the enemy’s country.  He should try to make his opponents fight where he wants them to fight, which will probably be as far as possible from his own territory and as near as possible to theirs.  Unless he can do this, invasion and even serious raids by him will be out of the question.  More than that, his inability to do it will virtually indicate that on its part the other side can fix the scene of active hostilities unpleasantly close to the points from which he desires to keep its forces away.

A line of ocean communications may be vulnerable throughout its length; but it does not follow that an assailant can operate against it with equal facility at every point, nor does it follow that it is at every point equally worth assailing.  Lines running past hostile naval ports are especially open to assault in the part near the ports; and lines formed by the confluence of two or more other lines—­like, for example, those which enter the English Channel—­will generally include a greater abundance of valuable traffic than others.  Consequently there are some parts at which an enemy may be expected to be more active than elsewhere, and it is from those very parts that it is most desirable to exclude him.  They are, as a rule, relatively near to the territory of the state whose navy has to keep the lines open, that is to say, prevent their being persistently beset by an enemy.  The necessary convergence of lines towards that state’s ports shows that some portion of them would have to be traversed, or their traversing be attempted, by expeditions meant to carry out either invasion or raids.  If, therefore, the enemy can be excluded as above mentioned, invasions, raids, and the more serious molestation of sea-borne commerce by him will be prevented.

If we consider particular cases we shall find proof upon proof of the validity of the rule.  Three great lines—­one from the neighbourhood of the Cape of Good Hope, one from the Red Sea, and a third from India and Ceylon—­converge near the south-western part of Australia and run as one line towards the territory of the important states farther east.  If an assailant can be excluded from the latter or combined line he must either divide his force or operate on only one of the confluents, leaving the rest free.  The farther he can be pushed back from the point of confluence the more