But towards the end of the third century, when the Pax Romana had long been established, there arose a new enemy—the Saxon sea-rovers—against whom new measures had to be taken. The first British fleet, which was formed under the leadership of Carausius, had such success against them that its commander seized the supreme power, and for a time maintained the independence of the island. But after the continental Empire resumed its sway in _A.D._ 296, this fleet was allowed to melt away; and a new land organisation was established, described in the _Notitia Imperii_, which split up the military command into three divisions, and placed them under the Prætorian Præfect of Gaul. By this arrangement, which may have originated in Constantine’s jealousy of his subordinates, the military force was chiefly massed in the two districts most exposed to invasion—the northern province, under the Duke of the Britains, whose headquarters were at York, and the south-eastern littoral, under the Count of the Saxon Shore. The central and western parts of the island, being much less exposed to danger, were defended only by a few squadrons of cavalry under the Count of the Britains, who had no legion under his command. It has generally been assumed that the northern province was bounded on the south by the Humber.[26] But if the “Saxon Shore” ended, as seems likely, at the eastern side of the Wash, such a division would result in leaving a long stretch of Lincolnshire Marsh, the flat shores of which were peculiarly exposed to invasion, outside the control of both the commanders, who had to defend the east coast. This district would thus be the “Achilles’ heel” of the whole system. Personally I am convinced that the northern province extended to the Wash, and that its southern boundary, which perhaps was ill-defined, was, roughly, the river Nene and Watling Street from High Cross (Venonæ) to Chester. It was essential for security that both banks of the Humber should be under one command; and it is hardly conceivable that Lincoln and York, whose connection by road and water was so intimate, were in different provinces. But though there was apparently no walled fortress on the hilly shore between the Tyne and the Humber, the exposed coast of Lindsey would naturally need some defence of the kind; and direct communication would, of course, be established between the northern province and the forces on the Saxon Shore.