It was in the year of our Lord 43, and in the reign of the Emperor Claudius, that the real conquest of Britain commenced under Aulus Plautius, and in seven years after the beginning of the war a part of the island had been reduced by that general and by his successor into the form of a province, and annexed to the Roman empire,—a result to which the valour and military talent of Vespasian, then serving under these generals, and afterwards Emperor, appears mainly to have contributed. In the year 50, under Ostorius, and perhaps his successor, the Roman province appears to have already extended to the Severn on the west, and to the Humber on the north.[28] Beyond its limits, on the west, were the warlike tribes of the Silures and the Ordovices, against whom the province was defended by a line of forts drawn from the river Sabrina or Severn, to a river, which cannot be identified with certainty, termed by Tacitus the Antona.[29] On the north lay the numerous and widely-extended tribes of the Brigantes, extending across the entire island from the Eastern to the Western Sea, and reaching from the Humber, which separated them from the province on the south, as far north, there seems little reason to doubt, as the Firth of Forth.[30] Beyond the nation of the Brigantes on the north, the Romans as yet knew nothing save that Britain was believed to be an island, and that certain islands termed Orcades[31] lay to the north of it; but the names even of the more northern tribes had not yet reached them. [Sidenote: A.D. 50. War with the Brigantes.] It is to the war with the Brigantes that we must mainly turn, in order to trace the progress of the Roman arms, and the extension of the frontier of the Roman province beyond the Humber. The Romans appear to have come in contact with the Brigantes for the first time in the course of the war carried on by Publius Ostorius, appointed governor of Britain in the year 50. That general had arrived in the island towards the end of summer; and the Barbarians, or those of the Britons still hostile to the Romans, believing he would not undertake a winter campaign, took advantage of his arrival at so late a period of the year to make incursions into the territory subject to Rome. Among these invading tribes were probably the Brigantes; but the general, by a rapid and energetic movement, put the enemy to flight, and it was on this occasion that the province was protected against the western tribes by a chain of forts. Having defeated the powerful nation of the Iceni, who endeavoured to obstruct his purpose by an attack from a different quarter, and who were destined at a subsequent period to place the Roman dominion in Britain in the utmost jeopardy, Ostorius reduced the tribes within the limits of the subjugated territory to entire obedience, and now turned his attention to more aggressive measures against those beyond its boundary.