There are many dimensions to the sea, and even a work of this size cannot possibly deal with them all. We have here the sea as a means of trade and communication, of the movement of people and ideas. We have the sea as a theatre of war, a sea fought-over itself, and the means of projecting force to distant lands. The sea unites, but it also divides; it may be an instrument of stability or of disorder. Different nations and societies, with different historical experiences, have seen the sea as a threat or an opportunity, and it is clear that either view may be correct. The sea has been the means of constructing empires, and of subverting them. The sea has opened opportunities for individuals to work and travel in remote parts of the world, and for whole populations to migrate to other continents. Fishing and whaling have provided rich sources of nutrition and industrial raw materials to peoples not otherwise endowed with them. Inland seas, navigable rivers and ship canals carry the sea into the interior of continents, but the influence and jurisdiction of coastal states may extend the control of the land far out to sea. Seaborne trade connects places on shore, often places remote from the coast; people and enterprises who have never seen the sea may still be deeply involved in the use of it. Nevertheless there have always been, and there remain, countries and regions which from choice or necessity have been little involved with the use of the sea. As a result they have avoided both the risks and the opportunities it brings.

In war the sea allows forces to be moved swiftly, economically and secretly to parts of the world which could not otherwise have been reached; it allows distant countries to unite their forces in empires or alliances, but it also parts friends and separates enemies, improving defence by distance and strategic depth. The free use of the sea invariably increases strategic options, but an uncommanded sea multiplies risks and dangers. For those who have failed to master it, the sea can be a destructive factor, dividing forces and exposing them to defeat in detail [Kennedy].

Because the sea means so much, it has a powerful symbolic force. A fleet of warships is both an instrument of power and an expression of wealth and might, of industrial strength and technical supremacy.