The study of war has taught its students for many centuries that a merely passive defense will finally be broken down, and that the most effective defense is the "offensive-defensive." Perhaps the clearest case of a correct offensive-defensive is Nelson's defense of England, which he carried on in the Mediterranean, in the West Indies, and wherever the enemy fleet might be, finally defeating Napoleon's plan for invading England--not by waiting off the coast of England, but by attacking and crippling Napoleon's fleet off the Spanish coast near Trafalgar.
The idea held by many people that the defense of a country can be effected by simply preventing the invasion of its coasts, is a little like the notion of uneducated people that a disease can be cured by suppressing its symptoms.

For even a successful defense of a coast against invasion by a hostile force cannot remove the inimical influence to a country's commerce and welfare which that hostile force exerts, any more than palliatives can cure dyspepsia.
Every intelligent physician knows that the only way to cure a disease is to remove its cause; and every intelligent military or naval man knows that history teaches that the only way in which a country can defend itself successfully against an enemy is to defeat the armed force of that enemy--be it a force of soldiers on the land, or a force of war-ships on the sea.

In naval parlance, "our objective is the enemy's fleet." If the duty of a navy be merely to prevent the actual invasion of its country's coasts, a great mistake has been made by Great Britain, France, and other countries in spending so much money on their navies, and in giving so much attention to the education and training of their officers and enlisted men.

To prevent actual invasion would be comparatively an easy task, one that could be performed by rows of forts along the coast, supplemented by mines and submarines.

If that is the only kind of defense required, navies are hardly needed.