For the moment an expeditionary force emerges from its own territorial waters--which may be any distance from a few miles up to many thousands of miles from the territorial waters of the adversary to be assailed--it must be prepared to defend itself, and naval force alone can afford it an adequate measure of defence. Military forces embarked in transports are defenceless and practically unarmed. They cannot defend themselves with their own arms, nor can the transports which carry them be so armed as to afford adequate defence against the smallest warship afloat, least of all against torpedo craft. Hence, unless the sea to be traversed has been cleared of the naval forces of the enemy beforehand, the invading military force must be covered by a naval force sufficient to overcome any naval force which the enemy is able to bring against it. If the latter can bring a fleet--as he must be able to do if the invasion is to be prevented--the covering fleet must be able to beat any fleet that he can bring. That condition being satisfied, however, it is clear that the covering fleet must be terribly hampered and handicapped in the ensuing conflict by the presence of a huge and unwieldy assemblage of unarmed transports filled with disarmed men, and by the consequent necessity of defending it against the attack of those portions of the enemy's naval force to which, albeit not suitable for engaging in the principal conflict, the transports would offer an otherwise defenceless prey. Hence the escorting fleet must be stronger than its adversary in a far larger proportion than it need be if naval issues pure and simple were alone at stake--so strong indeed that, if the transports were out of the way, its victory might be taken as certain. But if that is so it is manifest that the prospects of successful invasion would be immeasurably improved by seeking to decide the naval issue first--as Tourville very properly did in the Beachy Head campaign--and keeping the transports in hand and in port until it had been decided in favour of the intending invader. This is the eternal dilemma of invasion across a sea of which the command has not previously been secured. If you are not strong enough to dispose of the enemy's naval force you are certainly not strong enough to escort an invading force--itself helpless afloat--across the sea in his teeth. If you are strong enough to do this you will certainly be wise to beat him first, because then there will be nothing left to prevent the transit of your troops. In other words, command of the sea, if not absolutely and in all cases indispensable to a successful invasion, is at any rate the only certain way of ensuring its success.