If it be an instinct, it must have been implanted in us for a reason; and the reason is not hard to find in the universal law of self-protection, which cannot be satisfied with the ineffectual method of mere parrying or resisting.
Naval defense, like military defense, therefore, is not passive defense only, but contains an element of "offense" as well.

When the defense contains in large measure the element of offense, it is said in military parlance to be "offensive-defensive"; and the most effective defensive is this offensive-defensive.

When a defending force throws off its defensive attitude entirely and advances boldly to attack, it is said to have "assumed the offensive"; but even this assumption, especially if it be temporary--as when a beleaguered garrison makes a sortie--does not rob the situation of its defensive character.
For these reasons the dividing line between offense and defense is very vague; and it is made more vague through a realization by all military people that the offense has certain decided advantages over the defense (unless the defense has the advantage of position); so that when strained relations between two nations come, each is so fearful that the other will take the offensive first, when the two nations are near each other, that it is apt to take the offensive first--in real _self-defense!_ A striking illustration is the action of certain European Powers in the latter part of July, 1914.
In addition to the sincere convictions of either party, there is also apt to be considerable yielding to the temptation to persuade the world that the other party is the aggressor, merely to get the sympathy that usually goes to the innocent victim--the support of what Bismarck called "the imponderables." Few wars have been frankly "offensive," like the conquests of Alexander, Caesar, and Pizarro, at least in modern times; each side has usually claimed (and often sincerely believed) that its action was demanded in self-defense and that its cause was just.
To some in the United States naval defense means merely defense against invasion.

This notion is of recent growth, and certainly was not held by the framers of our Constitution.

Section 8 of Article I defines the powers of Congress; and although eight of the eighteen paragraphs deal exclusively with measures of defense on sea and land, only one of those paragraphs (the fifteenth) deals with invasion.
The, first paragraph reads: The Congress shall have power to lay and collect taxes, duties, imposts, and excises, _to pay the debts and provide for the common defense and general welfare of the United States_; but all duties, imposts, and excises shall be uniform throughout the United States.
The juxtaposition of the words "common defense" and "general welfare" in this admirably written paragraph could hardly have been accidental, or have been due to any other cause than a juxtaposition of those ideas in the minds of the Constitution's framers.