While Severus remained at Rome, after the defeat and death of Albinus, he received letters from the prefect of Britain announcing that the independent tribes had again broken loose and were in a state of open hostility, overrunning the province, driving off booty, and laying everything waste; and that it would be necessary for him either to send additional troops, or to come in person, to take steps for the protection of the province. The latter was the course adopted by Severus. Accompanied by his two sons, and from age and disease travelling in a litter, he arrived in Britain in the year 208, and drawing his troops together from all quarters, and concentrating a vast force, he prepared for war. His object in these great preparations was apparently not merely to repel the incursions of the enemy, but effectually to prevent them from renewing them by striking a severe blow, and carrying the war, as Agricola had done before him, into their fastnesses and the interior of the country. [Sidenote: Situation of hostile tribes.] When this war again drew the attention of the Roman historians to the state of the barbarian or hostile tribes, they found them in a very different situation from what they had been when so vividly painted by Tacitus, and so minutely described by Ptolemy. Instead of their condition as described by the former, who only knew them as a number of separate and independent tribes, inhabiting a part of Britain known by the name of Caledonia, and whom the imminence of the Roman invasion alone united into a temporary confederacy, they are now found combined into two nations, bearing the names respectively of ‘Caledonii’ and ‘Mæatæ,’ for into these two, says the historian Dio as abridged by Xiphiline, ‘were the names of the others merged.’ The nation of the ‘Mæatæ’ consisted of those tribes which were situated next the wall between the Forth and Clyde on the north. The ‘Caledonii’ lay beyond them. The former inhabited the more level districts, or, as the historian describes them, the plains and marshes, from which indeed they probably derived their name.[75] The latter occupied the more mountainous region beyond them. There is no reason to suppose that the line of separation between them differed very much from that which divided the tribe of the ‘Caledonii,’ as described by Ptolemy, from those on the south and east of them.