During the frequent wars with France from the commencement of the twelfth century onwards, the Channel acquired special significance from a military point of view, and it was from this time that the importance of “keeping the narrow seas” began to be recognised in English policy. The command of the Channel was not only of value in safeguarding the coast. The Channel formed the great avenue of commerce between the north and south of Europe. The merchant vessels from Venice, Genoa, and the Mediterranean, from Spain and France, passed northwards through it on their way to Flanders and the Baltic, and those from the Hanseatic towns and northern parts had in like manner to traverse it in their southern voyages. The Channel was thus crowded with shipping in summer, and the nation which commanded it had the power of interrupting the commerce of other nations, and consequently retained a potent political weapon in its hands. It is this aspect of “keeping the narrow sea” which forms the burden of the remarkable old poem, _The Libelle of Englyshe Polycye_. Moreover, in the period following the Norman Conquest another condition came into existence in connection with the security of the commerce passing through the Channel, which throws light on the origin of the English claim to sovereignty over it. As already mentioned, owing to the lawlessness that prevailed on the sea after the break-up of the Roman empire, when pirates and freebooters infested every coast, it became customary for merchants to associate themselves together for mutual protection. Their vessels sailed forth in fleets under the charge of an elected chief, called the “admiral,” and armed vessels were frequently fitted out by them for the express purpose of purging the sea of pirates. In the course of time this duty of maintaining the police of the seas was taken over by sovereign princes, who exercised their jurisdiction through an admiralty, and put in force the old “laws of the sea” which had gradually grown up among the merchant associations.[30] In the thirteenth century this supreme admiralty jurisdiction came to be regarded among the principal states of Europe as a prerogative of sovereign power, and it is about this time and in this connection that we first find certain evidence of the claim of England to the sovereignty of the adjacent sea. The Plantagenet kings, or at all events some of them, asserted the right of “maintaining the ancient supremacy of the Crown over the Sea of England” by exercising jurisdiction according to the old maritime laws, for the maintenance of “peace and justice amongst the people of every nation passing through the said sea.”[31] It was the production of the old rolls concerning these claims by Sir John Boroughs, the Keeper of the Records in the reign of Charles I., which furnished that king with the material on which to base his pretension to the sovereignty of the sea.