If the war is a purely naval war, that is, limited to the use of naval forces and hence directed solely to naval ends--as was the war between England and France, in the course of which the United States gained their independence--the command of the sea can theoretically be won by a single victorious battle. For if the main force of one side is destroyed, that belligerent becomes incapable of questioning the supremacy of the enemy, and hence must limit his sea action to sporadic attempts on communications. These can never be maintained to a degree that can be decisive, simply because a power greater than can be brought to the attack can be employed for their defence. Success in such a war, then, can simply be measured in terms of trade or of sea supply; defeat by the economic loss that its cessation must cause. There have been purely naval wars in the past and, could a combination be formed of countries whose aggregate sea-power was greater than that of Great Britain, a purely naval war might occur again. But it could only be brought about by such a conjuncture for the reason that Great Britain is the only country to which a purely naval defeat would mean such utter and immediate ruin, that her surrender to her sea conqueror would follow inevitably and promptly. This is so because, whereas almost every country is to some extent dependent upon sea supplies, Great Britain exists only in virtue of them. To us, therefore, the advantages that derive from possession of command of the sea are overwhelming; and our possession of it adversely to any other country must be disadvantageous, exactly in proportion as that country is dependent upon sea supplies.