Historical Overview : Prehistoric Britain to the Roman occupation pp. 23-24
Although Caesar Augustus planned to invade Britain at several different times during his reign, continued unrest in Gaul prevented him from carrying this out. In the year 43, an invasion force of 4 legions–about 25,000 troops–under the command of Aulus Plautius, Vespasian, and Claudius himself, entered Britain at Kent, soon taking London and Camulodunum (Colchester). Within a few years, fortifications were established at Lincoln, Wroxeter, Chester, Silchester, Old Sarum, Dorchester, and Wight; under Plautius’s successor Ostorius Scapula, assaults were made on Wales. Forces led by Suetonius Paulinus destroyed the rich culture at Anglesey in 60, but this was followed immediately by a large revolt of the Icenii in East Anglia, annihilation of the Roman forces quartered at Colchester, and the razing of London and St. Albans. Though outnumbered, Suetonius defeated the enemy in 61; over the next 20 years, the frontier was pushed farther north, and permanent outposts were built at Caerleon upon Usk, Chester, and York. Each of these was manned by one legion plus auxiliary regiments, securing the lowlands in central and southern Britain against attack from Wales and the highlands of Scotland. In 78, Julius Agricola began campaigns in southern Scotland, culminating in a major victory at Mons Graupius. The remaining 20 years of the first century saw consolidation of forces, refortification of the three legionary strongholds.
Sometime in the first decades of the second century, the legion stationed at York was apparently annihilated and replaced, possibly due to an unrecorded uprising during the reign of Hadrian. In 122, Hadrian himself visited Britain and established a new frontier line with the construction of a defensive wall from coast to coast. Antoninus Pius, Hadrian’s successor, came to quell insurrections in Britain in 138; a new wall was initiated in 139 and completed about 149. The Antonine Wall was breached and briefly abandoned during a British revolt in 155-58; it was breached again under Commodus ca. 180-192. Ulpius Marcellus won victories against the British in 184-85; in 197, Clodius Albinus took most of the Roman legionary force to Gaul to oppose Septimus Severus, a rival claimant for emperorship, abandoning the Hadrianic and Antonine defenses. The British penetrated the abandoned walls, wreaking widespread destruction as far south as York.
Under Severus in 208, the walls were recovered and rebuilt and Severus won major victories against the British. After his death at York in 211, his successor Caracalla completed the campaigns, quelling British unrest. The middle decades of the 3rd century were mostly quiet. Coastal raiding by Saxon pirates began in the latter half of the 3rd century, and from 287-93 Britain was ruled by a Belgian naval commander Carausius, who styled himself “emperor” of Britain and the coastal zone in the Low Countries. Carausius was murdered by his financial officer Allectus, who ruled then as his despotic successor until 296. Constantius Chlorus defeated Allectus, bringing Britain under Roman rule again, but Allectus’s depletion of forces in the north for his southern defenses allowed British entry from the north, and both York and Chester suffered renewed destruction.
During the fourth century, new fortresses were constructed at Wight and elsewhere; after a major assault by Picts, Scots, and Saxons from the north, Hadrian’s Wall was rebuilt a 3rd time under Theodosius. Client kingdoms were established in the north, making the need for a strong northern defense obsolete. Elsewhere in Europe, in 383 Magnus Maximus defeated Gratian at Paris, and the Roman Empire was divided, with Maximus ruling Gaul, Spain and Britain, Valentinian II ruling Italy, and Theodosius the Great ruling Constantinople. In 388 Maximus was killed in battle in Gaul against Theodosius, who ruled a reunified empire from 388-395.
Upon the death of Theodosius, in the early fifth century the empire was divided again. Stilicho, who directed the empire during the minority of Honorius, withdrew troops from Britain for the defense of Rome. The remaining administration in Britain appointed Constantine III their leader, but his campaigns in Gaul–like those of Maximus–ended in disaster. Appeals for reinforcement from Rome were unsuccessful, and following the execution of Constantine III, Honorius advised local administrators in Britain to fend for themselves.