Between the 7th and 5th centuries BCE, Celtic tribes from mainland Europe began migrating to the British Isles. Technically advanced compared to earlier inhabitants, they soon began to dominate political control and the culture of Britannia. Despite the fact that they were perhaps the most powerful and numerous people in much of Europe in 300 BCE, the Celts were very oriented towards varying tribes and were never able to build a unified Kingdom and prevent intertribal warfare. The total lack of political unity, despite their fierceness in battle, ultimately led to invasion and conquest by the Romans.
55 BCE saw the first of two consecutive Roman invasions from Gaul, under the command of Gaius Julius Caesar. These crossings, however, did not lead to any permanent occupation of the territory, but did open considerable trade between Rome and Britain. Rebellion in Gaul put an end to any thoughts Caesar may have entertained about conquest.
It would be nearly a century before the Romans would return. Discontent among the Britons, due to heavy taxes, jeopardized the iron trade that Rome had become so dependent on. In the year 43 CE an expedition was ordered against Britain by the Emperor Claudius, who sent his Legate, Aulus Plautius, and an army of 40,000 men. Within a generation most of southern Britain and Wales would be controlled by Rome. But the conquest was not fast or easy, the Catuvellauni King, Caratacus, resisted the occupation for 8 years and rebellion among the varying tribes was a constant threat. In 60 CE, the Iceni Queen Boudicea, (Boudica) led a revolt, that at first was devastating. Her armies burned 3 Roman Colonies including Londinium before she was eventually defeated by Seutonius Paulinus.
As a result of her defeat most of Southern Britannia was beginning to accept the reality of Roman occupation. The aggressive Romanization policy of Gnaeus Julius Agricola, between 78 and 84 CE added to this acceptance. He not only conquered most of Wales but pushed farther north into Caledonia (Scotland) than any Roman had ever been.
Roman success was short-lived, however. Heavy military defeats on the Danube forced the Romans to withdraw part of their army from Britain in 87 or 88, and as a result most forts beyond the Cheviots were abandoned. By the end of the century those bases in turn were given up and the most northerly Roman forts lay on the Tyne-Solway isthmus. The status quo was recognized by the Emperor Hadrian, who ordered the construction of his Wall on that line.
Hadrian's Wall took many years to build. Work probably started in 122 or 123 and the troops were still modifying the frontier installations at the time of the emperor's death in 138. Within a few months his successor, Antoninus Pius, decided on a new forward policy in Britain and preparations started in 139 with the recommissioning of the fort at Corbridge on one of the two main routes into Scotland.
When Rome had to withdraw one of its legions from Britain, the thirty-seven mile long Antonine Wall, connecting the Firths of Forth and Clyde, served temporarily as the northern frontier, beyond which lay Caledonia. The Caledonians, however were not easily contained; they were quick to master the arts of guerilla warfare against the scattered, home-sick Roman legionaries, including those under their ageing commander Severus. The Romans abandoned the Antonine Wall, withdrawing south of the better-built, more easily defended barrier of Hadrian, but by the end of the fourth century, the last remaining outposts in Caledonia were abandoned.
Despite the relative peace and prosperity of the South, Britannia was a frontier province and had continual trouble with the northern people like the Picts and Caledonians throughout the Roman occupation. Three full legions, Legio II Augusta, Legio VI Victrix and Legio XX Valeria Victrix and numerous axillaries were permanently stationed in the province. The massive Germanic migrations of the 5th century including Saxon invasions of Britain, eventually forced a complete withdrawal, leaving Roman citizens to fend for themselves.
The great mineral wealth of Britannia was among the primary reasons for Roman conquest. There were immense quantities of iron and tin which were of enormous importance to the Romans. Gold and silver were also available, which was badly needed to supplement nearly depleted mines in Hispania. Wool, livestock and grain rounded out the bulk of Britain's exports.
The Atrebates of southern Britain inhabited a region now contained within the modern county of Berkshire, and including the northern parts of Wiltshire, Hampshire and Surrey. Their tribal capital was situated at Calleva Atrebatum, today known as Silchester in Hampshire.
The Belgae were an amalgamation of Belgic states created by the Romans and initially ruled by the client-king Cogidubnus from Noviomagus [Chichester] on the south coast.
The Brigantes occupied the whole of northeast England with the exception of Humberside. They built small, fortified hill crofts, and a few forts. They were probably a loose federation of states.
The Cantiaci, or Cantii inhabited Cantium (Kent). Caesar thought them the most civilized tribe in Britain, and recorded four kings each ruling a minor kingdom; Cingetorix, Carvilius, Taximagulus and Segovax. They had a strong Belgic influence.
The Carvetii inhabited all of Cumbria and parts of north Lancashire, southwest Durham & southeast Dumfries and Galloway (Scotland). At first grouped with the Brigantes, they were later granted their own tribal council.
The Catuvellauni tribe was originally centered in Hertfordshire at Verulamium. Their territory was expanded in the years following Caesar to become the most extensive tribal canton in southern Britain.
The Coritani of the Leicester area were not a unified tribe, but a collection of like-minded peoples sharing the same outlook and social practices.
The Cornovii of Wroxeter and Shropshire had no tribal center prior to Roman times. They were remarkably aceramic, leading a mainly pastoral life.
The Deceangi had no self-governing institutions and no readily identified Romanized capital. They were probably under military government like their southern neighbors, the Ordovices, after the initial campaign of Agricola in AD 78. The extent of their tribal territories lay in the extreme northern coastal area of Wales; modern north-west and north-east Clwyd and northern Gwynedd.
The Demetae were from Carmarthen and Dyfedshire.
The Dobunni inhabited Cirencester and Gloucestershire. They were non-Belgic people occupying impressive hillforts with some Belgic influences.
The Dumnonii of Cornwall and Devon had no pre-Roman tribal center. They maintained strong traditions reaching back to the bronze age, but became civilized due to foreign interest in tin mines. They were also notably friendly to strangers, but fiercely combative when threatened.
The Durotriges lived in Dorset and Devon. They maintained an unusual density of powerful hillforts, and were fiercely independent baronies rather than a unified state.
The Iceni occupied Norfolk and northwest Suffolk and were a monarchic society state. Famous for a revolt against Roman rule led by their warrior queen, Boudicca.
The Ordovices were found mainly in south Gwynedd and south Clwyd. They had no self-governing institutions and no Romanized capital, and were probably under military government.
The Parisi inhabited Humberside and were surrounded to the north, west and southwest by the Brigantes, and by the Coritani to the south. They were rather more culturally advanced than the Brigantes, but inferior to the Coritani.
The Regni of Sussex and Hampshire were seemingly an amalgamation of Belgic peoples gathered together under the client king Cogidubnus by the Romans. The Kingdom did not exist prior to Roman rule.
The Silures could be found in Glamorgan and Gwent.
The Trinovantes occupied Essex and southern Suffolk. They were allies of Caesar and formed the Roman Colonia established at Camulodunum in 49 AD.