The River of London by Hilaire Belloc

The last historical aspect in which we must consider the River of London is the military aspect; and this aspect has three clearly marked divisions. We have first to consider the Lower Thames as an avenue of invasion; next as an obstacle to invasion of the northern part of the island, or to the passage of the troops from the north to the south. Thirdly, we must estimate what effect upon the military history of the country London itself has had, commanding as it does the lowest crossing of the Thames, always forming a vast base of supply, and for centuries by its stores and munitions forming an ally or an enemy of capital importance to whichever party it supported so long as warfare was waged in England.

The first two of these considerations are very sharply defined not only in their nature, but also in the historical periods over which each one predominated. The Thames as an avenue of ingress or invasion belongs to the earlier part of English history. The importance of the lower river in this category ends very nearly with the close of the Dark Ages. Its second character, that of an obstacle to the invader who would go northward or to troops marching from the north southward, in the main is discoverable of course throughout history, but chiefly belongs to the Middle Ages and the seventeenth century. I will take these three points, therefore, in their order and deal first with the invasions of England which the Lower Thames has served. These invasions from the first recorded case of the almost to the last may be grouped together under one description. They were the raids of the pirates.