S'il existe de nombreuses façons de penser et d'écrire l'Histoire, de se concentrer sur des sujets suffisamment restreints et précis pour être explorés de manière exhaustive au sein d'un seul article ou ouvrage est maintenant devenu la pratique courante des chercheurs qui doivent rendre compte d'une nouvelle recherche. Cela veut dire qu'un tel texte est généralement détaillé, parfois technique, et qu'il s'adresse à des collègues chercheurs, ou du moins suppose quelques connaissances préalables du sujet et de son contexte. Il s'appuiera systématiquement sur un appareil complet de notes indiquant les sources, et souvent sur des statistiques, des documents et d'autres preuves. Pour les lecteurs profanes ou les étudiants de premier cycle, les historiens adopteront un style différent, en balayant un spectre plus large du passé et en résumant leurs propres recherches et celles des autres. De tels textes sont conçus pour être plus faciles d'accès, mais s'appuient également sur leurs ouvrages de référence. Ces deux types de discours sur l'Histoire s'appuient solidement sur les preuves les plus sûres et les plus pertinentes qu'il soit. Il s'agit essentiellement d'une Histoire factuelle, de ce qui est connu avec certitude ou peut être déduit en toute confiance. Les conventions visant à garantir la rigueur intellectuelle des publications fournissent d'ailleurs un cadre formel qui limite délibérément les possibilités de spéculation. La signification profonde de l'Histoire, sa portée dans la société moderne, sont des réflexions laissées d'ordinaire à d'autre types d'écrits, voire à d'autres écrivains.

Les chercheurs(et les chercheuses) qui ont écrit pour le projet Océanides sont tous des autorités dans leur domaine. Mais il ne leur a pas été demandé de rédiger des articles universitaires, d'entreprendre de nouvelles recherches, ni de résumer l'état des connaissances sur un sujet particulier à la manière d'une encyclopédie. Leur mission a consisté à écrire des essais d'interprétation qui permettent d'explorer l'importance d'un sujet spécifique et d'une période historique donnée par le prisme d'une question simple. Dans son expression la plus claire, la question était la suivante: « Est-ce la mer qui a fait la différence? », ou « En quoi la mer a-t-elle changé les choses? » Ces questions sont simples, voire simplistes, mais elles ont pour but d'inciter les auteurs et les lecteurs à réexaminer la vaste portée des sujets proposés sur un mode que l'érudition académique et exhaustive ne permet généralement pas.

Le monde maritime revêt de nombreux aspects qui ne peuvent être abordés de manière exhaustive, même par un travail de cette envergure. La mer est par exemple un moyen d'échange commercial et de communication, elle permet la circulation des peuples et des idées. Elle est également le théâtre de la guerre; C'est un bien âprement disputé par lequel on peut envoyer des forces armées sur des terres lointaines. La mer unit autant qu'elle divise, C'est un instrument utilisé à des fins de stabilité ou de troubles.

Elle a été considérée comme un objet de menace ou comme une source d'opportunité par des nations et des sociétés diverses, avec des expériences historiques différentes. A l'évidence, ces deux opinions sont justes. La mer a été le moyen de construire des empires et de les détruire. Elle a offert la possibilité à chacun de travailler et de voyager dans des parties reculées du monde; et pour des peuples entiers de migrer vers d'autres continents. Des populations qui sans quoi auraient été démunis, ont pu profiter de la pêche et de la chasse à la baleine comme sources importantes d'alimentation et de matières premières industrielles. Les mers intérieures, les rivières navigables et les canaux maritimes ont transporté la mer au sein même des continents. Mais l'influence et la compétence territoriale des pays côtiers peuvent accroître le contrôle opéré par la terre sur la mer. Le commerce maritime relie entre eux des territoires terrestres qui sont souvent éloignés de la côte; des populations et des entreprises qui n'ont jamais vu la mer peuvent tout de même être profondément impliquées dans son utilisation. Néanmoins, il y a toujours eu et il existe encore des pays et des régions qui, par choix ou par nécessité, n'ont que peu de liens avec la mer. En conséquence, ils ont certes échappé aux risques qu'elle présente mais se sont également privés des opportunités qu'elle offre.

En temps de guerre, la mer permet aux troupes d'être déplacées rapidement, discrètement et à peu de frais dans des parties du monde qui n'auraient pu être atteintes autrement. Elle offre à des pays éloignés les uns des autres l'opportunité d'unir leur force et de créer des empires ou des alliances, mais elle éloigne également les alliés et sépare les ennemis, améliorant par là même la défense et par la distance et par la profondeur stratégique.

It is not entirely original to note that different kinds of social organisation fit naturally with different kinds of armed force. Aristotle, to look no further back, was clear that ‘light-armed troops and the navy are wholly on the side of democracy’, because the oarsmen of Greek warships were drawn from the poor free citizens, who had nothing to contribute to the war effort but their muscles, whereas the cavalry and infantry were recruited from the well-to-do who could afford horses, armour and weapons. It is worth the historian considering whether such a linkage can be established in later European history. Is it mere coincidence that in the English and Dutch cases naval power was associated with forms of representative government, while the great military powers (France, Prussia, Russia, Austria) were all autocratic monarchies? It is a matter of observation that army officers have traditionally been noblemen or gentlemen, while navies have tended to be run by middle-class professionals on whom the aristocratic concept of honour sat somewhat awkwardly. Is this coincidence, or is it in the nature of a navy to favour constitutional rather than autocratic government? If there is a connection, which is cause and which effect?

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.

There are many ways of thinking and writing about history, but the usual practice of scholars with new research to report is to concentrate on subjects sufficiently limited and precise to be thoroughly explored within the limits of a single article or book. This means that such writing is typically detailed and sometimes technical, addressed to fellow-scholars, or at least assuming some knowledge of the subject and its background. It will invariably be supported by a full apparatus of notes indicating the sources, and often by statistics, documents and other evidence. For general readers or undergraduates, historians will adopt a different style, covering a broader sweep of the past by summing-up their own and others' research. Such writing is designed to be more accessible, but it too will be supported by references to the published works on which it is based. Both sorts of history are firmly based on the most and best available evidence. This is fundamentally factual history, of what is known for certain or can be confidently inferred. The conventions intended to ensure intellectual rigour provide it with a formal structure which deliberately limits the scope for speculation. The deeper meaning of the history, its significance for modern society, is likely to be left for other writings, if not other writers.

The scholars who have written for the Océanides project are all authorities on their subjects, but they were not asked to write conventional scholarly articles, to undertake fresh research, nor to summarize all that is known on a particular subject in the style of an encyclopaedia. Their brief was to write interpretative essays exploring the significance of a particular subject and period of history through the lens of a single basic question. In its simplest form, the question was ‘was it the sea which made the difference?’, or ‘what difference did the sea make?’ These are simple questions, even simplistic, but they are meant to force authors and readers to reconsider the broad significance of their subjects in a way which conventional detailed scholarship does not usually do. Though the range of subjects and the styles of treatment vary a good deal, the fact that the same basic question is being put to each gives the whole work a common theme and unity.

John Hattendorf was born in Hinsdale, Illinois on 22 December 1941, a fortnight after the Imperial Japanese Navy had flung the United States abruptly into the Second World War. The next few years were to contribute a great deal of naval history to the United States and the world, but few would have searched for a future naval historian in the outer suburbs of Chicago, nor in Gambier, the small town amidst the peaceful Ohio cornfields where John studied as an undergraduate at Kenyon College. But Kenyon, though far from the sea, was not isolated from the wider world, and certainly not from the scholarly world. Charles Ritcheson, the noted historian of the American Revolution, was then a professor at Kenyon. His experience included wartime service in the US Navy, and a DPhil at Oxford, while his private interests ranged from the Paris Opera to the Beefsteak Club. John also had close contact with the distinguished German medievalist Richard G. Salomon, driven from his chair in Hamburg in 1934, who retired from Kenyon in 1962 aged seventy-eight but remained an active scholar. At one period when he was housebound after a fall, John fetched books for him from the library: a new parcel every week, a book for every day, each in a different language. Salomon introduced John to the scholarly tradition of the Monumenta Germaniae Historica, and aroused in him an enthusiasm for archives highly unusual in an undergraduate. Over two years’ voluntary work, John listed and organised the college archives, and he edited A Dusty Path, an anthology of documents and photographs drawn from them. Salomon almost lured him into medieval church history – but not quite.

Instead, on leaving Kenyon in 1964, John joined the US Navy, and the following day, as he remembers, ‘someone started a war in a place called Viet Nam’. In February 1965 he was commissioned as an ensign, and before the end of the year he was at sea off that coast as an officer of the destroyer USS O'Brien. In 1967 he was mentioned in despatches for his ‘skill and judgement’ under fire. From the O'Brienhe went to the Naval History Division in Washington, where he found himself in danger of a different sort.