Caribbean, it seems evident from the depth of the South Pass of the Mississippi, the nearness of New Orleans, and the advantages of the Mississippi Valley for water transit, that the main effort of the country must pour down that valley, and its permanent base of operations be found there. The defence of the entrance to the Mississippi, however, presents peculiar difficulties; while the only two rival ports, Key West and Pensacola, have too little depth of water, and are much less advantageously placed with reference to the resources of the country. To get the full benefit of superior geographical position, these defects must be overcome. Furthermore, as her distance from the Isthmus, though relatively less, is still considerable, the United States will have to obtain in the Caribbean stations fit for contingent, or secondary, bases of operations; which by their natural advantages, susceptibility of defence, and nearness to the central strategic issue, will enable her fleets to remain as near the scene as any opponent. With ingress and egress from the Mississippi sufficiently protected, with such outposts in her hands, and with the communications between them and the home base secured, in short, with proper military preparation, for which she has all necessary means, the preponderance of the United States on this field follows, from her geographical position and her power, with mathematical certainty. II. _Physical Conformation._--The peculiar features of the Gulf coast, just alluded to, come properly under the head of Physical Conformation of a country, which is placed second for discussion among the conditions which affect the development of sea power. The seaboard of a country is one of its frontiers; and the easier the access offered by the frontier to the region beyond, in this case the sea, the greater will be the tendency of a people toward intercourse with the rest of the world by it. If a country be imagined having a long seaboard, but entirely without a harbor, such a country can have no sea trade of its own, no shipping, no navy. This was practically the case with Belgium when it was a Spanish and an Austrian province. The Dutch, in 1648, as a condition of peace after a successful war, exacted that the Scheldt should be closed to sea commerce. This closed the harbor of Antwerp and transferred the sea trade of Belgium to Holland. The Spanish Netherlands ceased to be a sea power.