I. The Roman Conquest. - The conquest of Britain was undertaken by Claudius in A.D. 43. Two causes coincided to produce the step. On the one hand a forward policy then ruled at Rome, leading to annexations in various lands. On the other hand, a probably philo-Roman prince, Cunobelin (known to literature as Cymbeline), had just been succeeded by two sons, Caractacus (q.v.) and Togodumnus, who were hostile to Rome. Caligula, the half-insane predecessor of Claudius, had made in respect to this event some blunder which we know only through a sensational exaggeration, but which doubtless had to be made good. An immediate reason for action was the appeal of a fugitive British prince, presumably a Roman partisan and victim of Cunobelin's sons. So Aulus Plautius with a singularly well equipped army of some 40,000 men landed in Kent and advanced on London. Here Claudius himself appeared - the one reigning emperor of the 1st century who crossed the waves of ocean, - and the army, crossing the Thames, moved forward through Essex and captured the native capital, Camulodūnum, now Colchester. From the base of London and Colchester three corps continued the conquest.
The left wing, the Second Legion (under Vespasian, afterwards emperor), subdued the south; the centre, the Fourteenth and Twentieth Legions, subdued the midlands, while the right wing, the Ninth Legion, advanced through the eastern part of the island. This strategy was at first triumphant. The lowlands of Britain, with their partly Romanized and partly scanty population and their easy physical features, presented no obstacle. Within three or four years everything south of the Humber and east of the Severn had been either directly annexed or entrusted, as protectorates, to native client-princes.
A more difficult task remained. The wild hills and wilder tribes of Wales and Yorkshire offered far fiercer resistance. There followed thirty years of intermittent hill fighting (A.D. 47-79). The precise steps of the conquest are not known. Legionary fortresses were established at Wroxeter (for a time only), Chester and Caerleon, facing the Welsh hills, and at Lincoln in the northeast. Monmouthshire, and Flintshire with its lead mines, were early overrun; in 60 Suetonius Paulinus reached Anglesea. The method of conquest was the establishment of small detached forts in strategic positions, each garrisoned by 500 or 1000 men, and it was accompanied by a full share of those disasters which vigorous barbarians always inflict on civilized invaders. Progress was delayed too by the great revolt of Boadicea (q.v.) and a large part of the nominally conquered Lowlands. Her rising was soon crushed, but the government was obviously afraid for a while to move its garrisons forward. Indeed, other needs of the empire caused the withdrawal of the Fourteenth Legion about 67. But the decade A.D. 70-80 was decisive.
A series of three able generals commanded an army restored to its proper strength by the addition of Legio II. Adiutrix, and achieved the final subjugation of Wales and the first conquest of Yorkshire, where a legionary fortress at York was substituted for that at Lincoln.
The third and best-known, if not the ablest, of these generals, Julius Agricola, moved on in A.D. 80 to the conquest of the farther north. He established between the Clyde and Forth a frontier meant to be permanent, guarded by a line of forts, two of which are still traceable at Camelon near Falkirk, and at Bar Hill. He then advanced into Caledonia and won a "famous victory" at Mons Graupius (sometimes, but incorrectly, spelt Grampius), probably near the confluence of the Tay and the Isla, where a Roman encampment of his date, Inchtuthill, has been partly examined (see Galgacus). He dreamt even of invading Ireland, and thought it an easy task. The home government judged otherwise. Jealous possibly of a too brilliant general, certainly averse from costly and fruitless campaigns and needing the Legio II. Adiutrix for work elsewhere, it recalled both governor and legion, and gave up the more northerly of his nominal conquests. The most solid result of his campaigns is that his battlefield, misspelt Grampius, has provided to antiquaries, and through them to the world, the modern name of the Grampian Hills.
What frontier was adopted after Agricola's departure, whether Tweed or Cheviot or other, is unknown. For thirty years (A.D. 85-115) the military history of Britain is a blank. When we recover knowledge we are in an altered world. About 115 or 120 the northern Britons rose in revolt and destroyed the Ninth Legion, posted at York, which would bear the brunt of any northern trouble. In 122 the second reigning emperor who crossed the ocean, Hadrian, came himself to Britain, brought the Sixth Legion to replace the Ninth, and introduced the frontier policy of his age. For over 70 m. from Tyne to Solway, more exactly from Wallsend to Bowness, he built a continuous rampart, more probably of turf than of stone, with a ditch in front of it, a number of small forts along it, one or two outposts a few miles to the north of it, and some detached forts (the best-known is on the hill above Maryport) guarding the Cumberland coast beyond its western end. The details of his work are imperfectly known, for though many remains survive, it is hard to separate those of Hadrian's date from others that are later. But that Hadrian built a wall here is proved alike by literature and by inscriptions. The meaning of the scheme is equally certain.
It was to be, as it were, a Chinese wall, marking the definite limit of the Roman world. It was now declared, not by the secret resolutions of cabinets, but by the work of the spade marking the solid earth for ever, that the era of conquest was ended.
But empires move, though rulers bid them stand still. Whether the land beyond Hadrian's wall became temptingly peaceful or remained in vexing disorder, our authorities do not say. We know only that about 142 Hadrian's successor, Antoninus Pius, acting through his general Lollius Urbicus, advanced from the Tyne and Solway frontier to the narrower isthmus between Forth and Clyde, 36 m. across, which Agricola had fortified before him. Here he reared a continuous rampart with a ditch in front of it, fair-sized forts, probably a dozen in number, built either close behind it or actually abutting on it, and a connecting road running from end to end. An ancient writer states that the rampart was built of regularly laid sods (the same method which had probably been employed by Hadrian), and excavations in 1891-1893 have verified the statement. The work still survives visibly, though in varying preservation, except in the agricultural districts near its two ends. Occasionally, as on Croyhill (near Kilsyth), at Westerwood, and in the covers of Bonnyside (3 m. west of Falkirk), wall and ditch and even road can be distinctly traced, and the sites of many of the forts are plain to practised eyes. Three of these forts have been excavated.
All three show the ordinary features of Roman castella, though they differ more than one would expect in forts built at one time by one general. Bar Hill, the most completely explored, covers three acres - nearly five times as much as the earlier fort of Agricola on the same site. It had ramparts of turf, barrack-rooms of wood, and a headquarters building, storehouse and bath in stone: it stands a few yards back from the wall. Castle Cary covers nearly four acres: its ramparts contain massive and well-dressed masonry; its interior buildings, though they agree in material, do not altogether agree in plan with those of Bar Hill, and its north face falls in line with the frontier wall. Rough Castle, near Falkirk, is very much smaller; it is remarkable for the astonishing strength of its turf-built and earthen ramparts and ravelins, and for a remarkable series of defensive pits, reminiscent of Caesar's lilia at Alesia, plainly intended to break an enemy's charge, and either provided with stakes to impale the assailant or covered over with hurdles or the like to deceive him. Besides the dozen forts on the wall, one or two outposts may have been held at Ardoch and Abernethy along the natural route which runs by Stirling and Perth to the lowlands of the east coast.
This frontier was reached from the south by two roads. One, known in medieval times as Dere Street and misnamed Watling Street by modern antiquaries, ran from Corbridge on the Tyne past Otterburn, crossed Cheviot near Makendon Camps, and passed by an important fort at Newstead near Melrose, and another at Inveresk (outside of Edinburgh), to the eastern end of the wall. The other, starting from Carlisle, ran to Birrens, a Roman fort near Ecclefechan, and thence, by a line not yet explored and indeed not at all certain, to Carstairs and the west end of the wall. This wall was in addition to, and not instead of, the wall of Hadrian. Both barriers were held together, and the district between them was regarded as a military area, outside the range of civilization.
The work of Pius brought no long peace. Sixteen years later disorder broke out in north Britain, apparently in the district between the Cheviots and the Derbyshire hills, and was repressed with difficulty after four or five years' fighting. Eighteen or twenty years later (180-185) a new war broke out with a different issue. The Romans lost everything beyond Cheviot, and perhaps even more. The government of Commodus, feeble in itself and vexed by many troubles, could not repair the loss, and the civil wars which soon raged in Europe (193-197) gave the Caledonians further chance. It was not till 208 that Septimius Severus, the ablest emperor of his age, could turn his attention to the island. He came thither in person, invaded Caledonia, commenced the reconstruction of the wall of Hadrian, rebuilding it from end to end in stone, and then in the fourth year of his operations died at York. Amid much that is uncertain and even legendary about his work in Britain, this is plain, that he fixed on the line of Hadrian's wall as his substantive frontier. His successors, Caracalla and Severus Alexander (211-235), accepted the position, and many inscriptions refer to building or rebuilding executed by them for the greater efficiency of the frontier defences.
The conquest of Britain was at last over. The wall of Hadrian remained for nearly two hundred years more the northern limit of Roman power in the extreme west.