Whether in peaceful trade or warlike attack, the sea unites more than it divides. Even if it were possible to treat England, or the British Isles, as a single, homogenous, united nation, it would still be impossible to write its naval history without reference to the histories of the other nations, near and far, with which the sea has connected it.

It is not surprising that only one medieval state, Venice, long possessed anything clearly identifiavble as a navy in this sense. We shall see that no state in the British Isles attained attained this level of sophistication before the 16th century, and no history of the Royal Navy, in any exact sense of the words, could legitimately begin much before then. This book, which does, is not an institutional history of the Royal Navy, but a history of naval warfare as an aspect of national history. All and any methods of fighting at sea, or using the sea for warlike purposes, are its concern.

'A Naval History of Britain' which begins in the 7th century has to explain what it means by Britain. My meaning is simply the British Isles as a whole, but not any particular nation or state or our own day... 'Britain' is not a perfect word for this purpose, but 'Britain and Ireland' would be both cumbersome and misleading, implying an equality of treatment which is not possible. Ireland and the Irish figure often in this book, but Irish naval history, in the sense of the history of Irish fleets, is largely a history of what might have been rather than what actually happened.

It is well known that we fight in God's cause... but unless God helps us by a miracle the English, who have faster and handier ships than ours, and many more long-range guns, and who know their advantage just as well as we do, will never close with us at all, but stand aloof and knock us to pieces with their culverins, without our being able to do them any serious hurt. So we are sailing against England in the confident hope of a miracle.

Its subject is the slow and erratic process by which the peoples of the British Isles learnt - and then for long periods forgot - about the 'Safeguard of the Sea', as the 15th century phrase had it, meaning the use of the sea for national defence, and the defence of those who used the sea.

The common sense of the word (navy) as we use it today refers to a permanent fighting service made up of ships designed for war, manned by professionals and supported by an adminsistrative and technical infrastructure. A navy in this sense is only one possible method of making war at sea, and by some way the most difficult and the most recent. There have in the past been, and to some extent still are, many other ways of generating sea power.

In every age states of varying size and constitution and at every level of development have found naval warfare to be one of their most formidable and expensive tasks. Ships have always been large, costly and complicated, and warships much more complicated and costly than any others. Scholars are nowadays inclined to emphasize the power, wealth and sophistication of the Anglo-Saxon kingdoms, and there is not more striking illustration of this than the advanced and elaborate administrative structures of the early English navy.