We have thus the two centres round which revolve all the military study of the Caribbean Sea and the Gulf of Mexico. The two sheets of water, taken together, control or affect the approaches on one side to these two supreme centres of commercial, and therefore of political and military, interest. The approaches on the other side--the interior communications of the Mississippi, that is, or the maritime routes in the Pacific converging upon the Isthmus--do not here concern us. These approaches, in terms of military art, are known as the "communications." Communications are probably the most vital and determining element in strategy, military or naval. They are literally the most radical; for all military operations depend upon communications, as the fruit of a plant depends upon communication with its root. We draw therefore upon the map the chief lines by which communication exists between these two centres and the outside world. Such lines represent the mutual dependence of the centres and the exterior, by which each ministers to the others, and by severance of which either becomes useless to the others. It is from their potential effect upon these lines of communication that all positions in the Gulf or the Caribbean derive their military value, or want of value. It is impossible to precede or to accompany a discussion of this sort with a technical exposition of naval strategy. Such definitions of the art as may be needed must be given _in loco_, cursorily and dogmatically. Therefore it will be said here briefly that the strategic value of any position, be it body of land large or small, or a seaport, or a strait, depends, 1, upon situation (with reference chiefly to communications), 2, upon its strength (inherent or acquired), and, 3, upon its resources (natural or stored). As strength and resources are matters which man can accumulate where suitable situation offers, whereas he cannot change the location of a place in itself otherwise advantageous, it is upon situation that attention must primarily be fixed. Strength and resources may be artificially supplied or increased, but it passes the power of man to move a port which lies outside the limits of strategic effect. Gibraltar in mid-ocean might have fourfold its present power, yet would be valueless in a military sense.