But when the matter is more carefully examined in its historical aspects, a less selfish explanation may be found of the tendency to appropriate seas in the middle ages. In the state of wild anarchy which prevailed after the break-up of the Roman empire, pirates swarmed along every coast where booty might be had. Scandinavian rovers infested the Baltic, the North Sea, and the Channel; Saracens and Greeks preyed upon the commerce of the Mediterranean; everywhere the navigation of trading vessels was exposed to constant peril from the attacks of freebooters. The sea was then common only in the sense of being universally open to depredation.[2] The lawlessness and insecurity that reigned on the sea led merchants, in the absence of effective sovereign authority, to form associations among themselves for mutual protection, and to maintain by force the security of navigation in the common interest. Independent princes at first made use of the armed fleets of those voluntary associations, and later, as their power grew stronger and better organised, they took over the duty of policing the neighbouring seas under an admiralty jurisdiction of their own, which enforced the maritime laws and customs, such as the Laws of Oleron, that had been gradually developed among the merchant associations. In the thirteenth century this duty of exercising supreme admiralty jurisdiction on the neighbouring sea came to be regarded as a prerogative of sovereign power,[3] and it was only a short step further to the assertion of an exclusive dominion. It was natural that this assumption of sovereignty on the sea should first be made by the great trading cities of Italy, who then controlled the important traffic between the east and the west, and whose shipping was to be found in all the ports of Christendom. It was also natural that the Italian jurists should be the first to attempt to give it a legal sanction, by assigning a large part of the bordering sea for the exercise of those sovereign functions which were originally confined to the maintenance of order and the punishment of delinquents. There is little doubt that the assumption of sovereign jurisdiction in this way was advantageous to navigation and commerce in those times, though later, with the extension of commercial intercourse and the increased security of the sea, it became burdensome and unnecessary.