From this time those walls become the principal object in the British history.  The Caledonians, or (as they are called) the Picts, made very frequent and sometimes successful attempts upon this barrier, taking advantage more particularly of every change in government, whilst the soldiery throughout the Empire were more intent upon the choice of a master than the motions of an enemy.  In this dubious state of unquiet peace and unprosecuted war the province continued until Severus came to the purple, who, finding that Britain had grown into one of the most considerable provinces of the Empire, and was at the same time in a dangerous situation, resolved to visit that island in person, and to provide for its security.  He led a vast army into the wilds of Caledonia, and was the first of the Romans who penetrated to the most northern boundary of this island.  The natives, defeated in some engagements, and wholly unable to resist so great and determined a power, were obliged to submit to such a peace as the emperor thought proper to impose.  Contenting himself with a submission, always cheaply won from a barbarous people, and never long regarded, Severus made no sort of military establishment in that country.  On the contrary, he abandoned the advanced work which had been raised in the reign of Antoninus, and, limiting himself by the plan of Adrian, he either built a new wall near the former, or he added to the work of that emperor such considerable improvements and repairs that it has since been called the Wall of Severus.

Severus with great labor and charge terrified the Caledonians; but he did not subdue them.  He neglected those easy and assured means of subjection which the nature of that part of Britain affords to a power master of the sea, by the bays, friths, and lakes with which it is everywhere pierced, and in some places almost cut through.  A few garrisons at the necks of land, and a fleet to connect them and to awe the coast, must at any time have been sufficient irrecoverably to subdue that part of Britain.  This was a neglect in Agricola occasioned probably by a limited command; and it was not rectified by boundless authority in Severus.  The Caledonians again resumed their arms, and renewed their ravages on the Roman frontier.  Severus died before he could take any new measures; and from his death there is an almost total silence concerning the affairs of Britain until the division of the Empire.

Had the unwieldy mass of that overgrown dominion been effectively divided, and divided into large portions, each forming a state, separate and absolutely independent, the scheme had been far more perfect.  Though the Empire had perished, these states might have subsisted; and they might have made a far better opposition to the inroads of the barbarians even than the whole united; since each nation would have its own strength solely employed in resisting its own particular enemies.  For, notwithstanding the resources which might have been expected from the entireness of so great a body, it is clear from history that the Romans were never able to employ with effect and at the same time above two armies, and that on the whole they were very unequal to the defence of a frontier of many thousand miles in circuit.