equent
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.

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,