Britain Britain probably became known to the Mediterranean world in the fifth century BC through the Phoenicians of Carthage, the great traders and voyagers of the ancient world, because south-west Britain was one of the few sources of tin, a necessary component of bronze very scantily diffused in the Mediterranean area. Greeks of the fifth century heard only vague stories about the ‘Tin Islands’ (Cassiterides), but in the late fourth century a Greek navigator from Massilia (Marseilles), Pytheas, visited Cornwall and the tin-depot at St Michael's Mount on his circumnavigation of Britain. By the second century BC regular trade routes between Britain and the Mediterranean were established.
Julius Caesar invaded Britain in expeditions in 55 and 54 BC, perhaps feeling that his conquest of Gaul was hardly secure without at least a show of Roman strength in Britain (Britons fought in the Gallic armies and rebellious Gauls sought refuge in Britain); total conquest may not have been his intention. The first invasion was in the nature of a reconnaissance, but the second was a more serious affair, involving a fleet of over 800 vessels and probably more than 20, 000 men, including 2, 000 cavalry. Caesar crossed the river Thames (Tamesis or Tamesa) to attack the British commander Cassivellaunus, king of the Catuvellauni, the most powerful of the southern tribes. Cassivellaunus was forced to give hostages and pay tribute, then Caesar withdrew to Gaul, where he feared an uprising.
Britain was now a recognized part of the Roman world, though Rome was not to invade again for another century. In the thirties and twenties BC the Latin poets Virgil and Horace sometimes speak as if the emperor Augustus was on the verge of annexing Britain, but nothing came of it and it was the emperor Claudius who made the conquest, sending a force of perhaps 40, 000 men in AD 43; he arrived in person for the capture of Camulodunum—Colchester—although whether he played an active part in the fighting is disputed. Claudius pursued the policy of having client kingdoms around the province: such were the Iceni of Norfolk and the Brigantes in the north, though the latter felt themselves to be largely independent. An important client king was Cogidubnus, whose land centred on the Chichester area of west Sussex (the palace excavated at Fishbourne is almost certainly his). Aulus Plautius, who had led the invasion, was made governor of the province with orders to secure the rest of the country and by 47, when his term as governor ended, the whole area south of the Humber and east of the Severn was under Roman control. Ostorius Scapula, governor from 47 to 52, made advances into Wales, and in 51 defeated Caratacus, who had become the leader of British opposition to Rome. Gaius Suetonius Paulinus, governor from 58 to 61, quashed the much more serious revolt of Boudicca, ushering in a long period of stability in the south. In the 70s the whole of Wales was subdued, and Agricola (governor 77/8–84) advanced into Scotland, reaching the line of the Forth and the Clyde in 81. In 83 he moved further north and in 84 inflicted a famous defeat on the Caledonians at Mons Graupius. He sent a fleet around the north of Britain, and thus confirmed that it was an island. By the end of the century, or soon after, however, Scotland beyond the line of the Forth and the Clyde was abandoned, and the opportunity never came again to embrace the whole island in one province.
Under the emperor Trajan (98–117) there seems to have been a withdrawal from the south of Scotland and a new boundary created along the line between the Solway and the Tyne, confirmed by the subsequent emperor Hadrian (117–138) with the most elaborate frontier in the Roman world—‘Hadrian's Wall’—which was begun after the emperor's visit in 122, and ran from Wallsend-on-Tyne in the east to Bowness-on-Solway in the west, 118 km. (74 miles) in all. It acted as a base for patrols, cut off raiders from the north, and produced settled conditions in the south in which the economy and the arts of peace could flourish.
Soon after Antoninus Pius became emperor in 138 he initiated a sudden change of policy. The governor of Britain, Q. Lollius Urbicus, advanced once more into southern Scotland, reconquered the lowlands, and built a new frontier wall of turf—the Antonine Wall—across the line of the Forth and the Clyde, 60 km. (37 miles) long, less elaborate than Hadrian's Wall but of the same basic plan, with forts interspersed with smaller stations. The history of this period is confused, but the Antonine Wall seems to have been abandoned in c.163. Not long after 180 what is described as the greatest war of the emperor Commodus' reign occurred in Britain. Tribes from the north swept over Hadrian's Wall, ravaged widely, and destroyed a Roman force. A punitive campaign was successfully waged in 184, followed by withdrawal to the Wall. Roman influence was accepted in the lowlands but the tribes remained autonomous.
After the assassination of the emperor Commodus in 192, Clodius Albinus, governor of Britain, became a rival to the new emperor, Septimius Severus, and eventually claimed the throne. In 196 he crossed with an army to Gaul, where he was defeated by Severus in 197 and committed suicide. His withdrawal of troops from Britain gave the opportunity to those who hated Rome, especially those beyond the Wall, to vandalize and loot. In 208 Severus himself arrived in Britain and directed a punitive expedition into Scotland; he was planning a further campaign when he died in 211. At some time in the early third century Britain was divided into two provinces, the northern having its centre at York and the southern at London. For the rest of the century it remained relatively undisturbed and moderately prosperous, untypically so if it is compared with the convulsions elsewhere in the empire in this period.
In 286 or 287 Carausius, commander of the British fleet, declared himself emperor of Britain, making the island independent of the central government in Rome. Carausius was left in peace and his government was efficient and successful, but in 294 he was assassinated by one of his chief associates, Allectus, who himself seized the provinces of Britain. In 296 Constantius launched an invasion against Allectus, and the latter was killed in battle. Constantius thereby regained the provinces, which needed considerable reconstruction. In 306, when Constantius had become senior Augustus (emperor), he was joined by his son Constantine for another invasion of Scotland, and won a great victory. Shortly after, he died at York and his son succeeded as Caesar. Constantine had soon to leave for Gaul, but during the first half of the fourth century Britain enjoyed peace and prosperity.
In 337 Constantine died and out of the ensuing chaos his son Constantine II emerged as senior Augustus with command over Britain, Gaul, and Spain. He invaded Italy, the domain of his brother Constans, in 340, and was killed in battle. Some unknown crisis brought Constans unexpectedly to Britain in 343, perhaps attacks by Picts and Scots beyond the Wall and by Saxon and Frankish marauders in the south. By 368 the threat to British security was alarming, and in that year a concerted attack was made by the tribes in the north, the west coast, and the south-east; the country was over-run and London threatened. It was two years before a military commander Theodosius (the Elder), a Spaniard, sent by the emperor Valentinian, could restore order. He drove out the invaders and again repaired the Wall. Once more Britain had a firm government and secure defences, but the growing power of barbarians outside the empire threatened all Europe and from 383 there were repeated withdrawals of troops from Britain to defend Italy. Soon after 400 effective occupation of Hadrian's Wall had ceased, but there was no sudden conclusion to Roman rule in Britain—rather it was a gradual process spread over several decades. By c.450 the barbarians were in Britain to stay.