In the Caribbean, likewise, the existence of numerous important ports, and a busy traffic in tropical produce grown within the region itself, do but make more striking the predominance in interest of that one position known comprehensively, but up to the present somewhat indeterminately, as the Isthmus.  Here again the element of decisive value is the crossing of the roads, the meeting of the ways, which, whether imposed by nature itself, as in the cases before us, or induced, as sometimes happens, in a less degree, by simple human dispositions, are prime factors in mercantile or strategic consequence.  For these reasons the Isthmus, even under the disadvantages of land carriage and transshipment of goods, has ever been an important link in the communications from East to West, from the days of the first discoverers and throughout all subsequent centuries, though fluctuating in degree from age to age; but when it shall be pierced by a canal, it will present a maritime centre analogous to the mouth of the Mississippi.  They will differ in this, that in the latter case the converging water routes on one side are interior to a great state whose resources they bear, whereas the roads which on either side converge upon the Isthmus lie wholly upon the ocean, the common possession of all nations.  Control of the latter, therefore, rests either upon local control of the Isthmus itself, or, indirectly, upon control of its approaches, or upon a distinctly preponderant navy.  In naval questions the latter is always the dominant factor, exactly as on land the mobile army—­the army in the field—­must dominate the question of fortresses, unless war is to be impotent.

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.