In AD 378 the Romans suffered a catastrophic defeat at Adrianople where two-thirds of their eastern army was destroyed. Troops had to be brought from the west, which included those in Britain. In addition provincial barbarians led by their own kings and chieftains filled the ranks. In Britain there were campaigns against the Picts and the Scots led by Magnus Maximus, a Spaniard, who had been with Theodosius in Britain in AD 367–9 and had been sent back to organize the province’s defences. He was successful in these campaigns but was resentful that he had not been promoted to higher office. In AD 383, making himself popular with the troops and taking advantage of their resentment against Gratian, he got himself acclaimed emperor. He left for Gaul, taking with him a large number of troops from Britain, probably from some of the forts in Wales and the northern Pennines that were now abandoned. When he arrived in Gaul, he was joined by some of the troops in Germany. Gratian confronted him in battle but many of his troops deserted to Maximus so that he was forced to flee towards the Alps. Maximus sent his cavalry officer Andragathius after Gratian, who was caught and killed, and forced Theodosius (the son of Count Theodosius and emperor of the east since 379) to accept him as emperor in the west, where he proved his worth by holding Gaul against the barbarian invasions. This did not satisfy him and he invaded Italy in AD 387 driving Valentinian, who still had vestiges of his rule there, out of Italy to seek refuge with Theodosius in Constantinople. Theodosius was then forced to intervene and a decisive encounter took place in AD 388 at Aquileia where Magnus was defeated and executed. In AD 394 Theodosius had to intervene again in Italy in order to counter an invasion of the Goths. In this he was successful and managed to unite the empire, but he died in January 395 and the empire was divided between his two sons, Honorius and Arcadius, both already acclaimed Augustus. Honorius took the western empire, reigning from AD 395 to 423, a considerable length of time for an emperor, but over that time he had to face a series of crises, some of which affected Britain and included its loss as a province.
Gildas, writing in the sixth century AD, indicated that Magnus had removed so many troops from Britain that the Picts and the Scots were able to raid Britain in huge numbers. The Irish made raids along the west coast from Cumbria to Wales. They attacked inland as far as Wroxeter and then began to settle in Wales, possibly as a result of military weakness due to the removal of troops from this area. Legion XX had probably been withdrawn from Chester about this time, as was Legion II Augusta from Caerleon. Small garrisons of auxiliaries seem to have remained in some forts in Wales, including at Forden Gaer and Caernarfon, but it would seem that the Picts, unchallenged, raided as far as the south coast.
Gildas said that the Britons, ‘promising unwavering and whole-hearted submission to Roman rule, if only the enemy could be kept at a greater distance’, continually implored Flavius Stilicho, a Vandal general, to send an expedition to help them. He was married to Theodosius’s niece Serena, and was the power behind the throne of the young Honorius. By now the Roman military command was relying on the support of those barbarians who had once been despised. Large companies of Franks and Alamanni had become part of the army in the west and barbarian names were now common in civilian and military commands. Many of these barbarian groups cooperated on a purely cash basis, and the Romans had to accept this partly because of their own shortage of manpower and partly because the Roman nobility declined to give either money or potential recruits from their estates. This, however, resulted in a certain tension, as the Romans were never able to have full confidence in these new mercenaries and allies.
Claudian, Stilicho’s court poet, indicated that Stilicho ‘had a care’, which ensured ‘Britain should not fear the spears of the Scots nor tremble at the Picts’; according to Gildas, Stilicho did send help (a ‘legion’) with the result that many of the invaders were killed, which seemed to provide some respite. Possibly some troops were still able to provide a defence. He also reported that another mission was sent to help Britain but as he also mentioned that the Britons were ordered to build a wall ‘from sea to sea between cities, which happened to have been placed there through fear of the enemy’, it is not clear if his accounts are accurate. The wall referred to must be Hadrian’s Wall and the ‘cities’ are presumably the forts. The original building of the Wall had been lost in antiquity and Gildas was probably trying to explain when it was built. His statements might be explained by crude inscriptions found in the Wall, which indicate that construction units were provided by the civitates of the Durotriges and the Catuvellauni. These may have been fighting units transferred to the north to repair the frontier and reinforce its garrison.
Rome, however, was more concerned with other barbarian invasions. The empire was being menaced elsewhere and in AD 401 more troops, mostly from the forts in Wales and the Pennines, were withdrawn to help stop the advances of Alaric, leader of the Visigoths. From then on there were successive withdrawals so that Britain was denuded of troops, which led to more attacks on Britain. Irish attacks on the south coast by the Irish king, Niall of the Nine Hostages, may be dated to AD 405 but before that there is evidence of the destruction and burning of villas in the Somerset and Gloucestershire regions, probably by Irish raiders. Keynsham villa was burned about AD 378, Kings Weston about AD 384; and Atworth, Box, Colne, North Wraxall and others in those areas suffered the same fate.
St Jerome, writing about AD 415, stated that Britain was a ‘province fertile in tyrants’, and this seems to have been the case. There were a succession of usurpers of which the first was Marcus, who was elected by the army in AD 406, but within a year he had been deposed and killed. In AD 407 Gratian, described as ‘a citizen of Britain’, was elected and as quickly dispatched. The army then elected a soldier, who took the name Constantine III, probably believing that this name would help him to achieve the empire. The fifth-century historian Orosius said that he was ‘elected from the lowest ranking soldiers, solely because of the hope attributed to his name and not because he had achieved any honour’. He proved, however, to be an effective military leader. Despite the previous withdrawal of troops he was able to take even more troops from Britain, possibly attracted by booty and adventure, and crossed with them to Gaul.
Events, however, had overtaken the empire. On a bitterly cold 31 December 406, when the Rhine was frozen, a vast number of Alans, Sueves and Vandals crossed the river and spread across northern Gaul. Constantine took advantage of this, quickly established an administration in Gaul, and then began to bring the invaders under control. He was not entirely successful but it was sufficient to ensure his authority. He then sent his son Constans and his general Gerontius south to invade Spain. By AD 408 Spain was in his grip. He had won support because he realized that the best chance of defending the west lay in strong government in Gaul, Spain and Britain. Honorius, in AD 409, also accepting the inevitable, recognized the validity of Constantine’s rule, proclaiming him as Augustus and seemingly agreeing to a united Gallic–Britannic province with Constantine as a legitimate emperor.
Problems in Italy were to devastate this arrangement. In AD 410 an alliance between Alaric, leader of the Visigoths, and Honorius broke down. Alaric led his force into Italy and sacked Rome, with his Gothic troops doing the greater damage. Alaric died the next year but this did not spare Rome as his brother Athaulf led another army into Italy leading to confusion and tumult which resulted in Honorius’s loss of confidence in Stilicho and his subsequent execution. Meanwhile Constantine was losing control of events in Gaul. He had recruited barbarian troops into his army but, when German barbarians crossed into Gaul, they failed to oppose them, instead concentrating on plunder. Constans’s troops in Spain also got out of control. He blamed Gerontius who promptly rebelled and supported a soldier, Maximus, as a rival emperor. He allied himself with barbarian invaders who captured and murdered Constans. They then moved into Gaul at the same time as Constantine, wanting more power, was leading his forces into Italy. Hearing this Constantine returned to Gaul but was besieged in Arles by Gerontius. At the same time the Burgundians invaded Gaul with the intent of settling there.
Constantine’s empire was disintegrating and his British forces were losing faith in him. Whether he knew it or not there were also serious attacks on Britain, which led to the Britons withdrawing their support from Constantine. Zosimus described them as ‘throwing off Roman rule and living independently, no longer subject to Roman law and reverting to their native customs and setting up their own administration as well as they could’. This indicated that they expelled the Roman administrators, which was to have serious consequences later.
Zosimus said that Honorius sent letters to the cities of Britain, bidding them to fend for themselves, which implies that they had appealed to him for help. Any attempt to help was now out of Constantine’s control. Honorius’s army advanced to Arles, defeated Gerontius’s forces who were besieging the city and forced Constantine to surrender. Constantine was executed and Gerontius escaped to Spain where some of his troops, hearing of his defeat, besieged his house. Realizing there was no escape and yielding to the entreaties of his wife he beheaded her and committed suicide. This Gallic Empire had now been destroyed.
Honorius seemed to have made no attempt to bring Britain back under Roman control. He had no troops available to do this and was concerned with containing events in Gaul where the real power was in the control of the Burgundians and the Visigoths. In fact, Procopius, a sixth-century historian who was prefect of Constantinople during Justinian’s reign, said that the Romans were never able to recover Britain, which from then on remained on its own, subject to various usurpers (tyrants).
It would seem therefore that from AD 410 the Britons had to rely on their own precautions against any raiders. Direct Roman rule in Britain had ceased to exist, brought on by a succession of rebellions against the central authority. There was no withdrawal of Roman authority. Britain had gradually withdrawn from Rome. Britain, on the extreme north-west of the Roman Empire, may never have been fully assumed into that empire possibly because the whole population have never been fully Romanized. Celtic tribal authority was allowed to continue when towns became civitas capitals. Britons in remote areas continued to follow their own way of life. It was in the towns and the villas that people had become most attracted to conditions that seemed to offer a better way of life.
Towns hoped that stout walls would protect them; their citizens might raise a militia or hire mercenaries. That this was possible for some towns is shown by a visit of Germanus, Bishop of Auxerre, to Verulamium in AD 429. According to the Gallic cleric Constantius of Lyon, Germanus had been sent to Britain to counter the Pelagian heresy. This has been spread by Pelagius, a Briton, who decreed that man was responsible for his own actions and by his own free human will and God-given nature would determine his own salvation. This was in direct opposition to St Augustine’s view that man was utterly dependent on the divine will and the grace of God, because the frail nature of his being makes him unable to achieve grace and salvation by himself.
The Pelagian heresy gained a strong hold on the upper classes in Britain and it may have been this belief that helped them to take matters into their own hands for their defence – to exercise, in fact, free will. After Germanus’s arrival his preaching seemed to have checked further spread of these heretical opinions. He then visited the shrine of St Alban at Verulamium to hold an assembly, which suggests that the town then had some form of government to organize this. This is confirmed by the fact that he healed the daughter of a man having tribune power, that is, a man having military leadership in the Roman sense. Shortly after, Germanus won a victory over a raid by the Picts and the Scots, by leading the Britons into battle and urging them to cry ‘Alleluia’ at the moment of attack, an action which went back to the Celtic custom of shouting a battle cry when they attacked enemy forces. Constantius also states that Germanus made a second visit to Britain in AD 437 but, as he was involved in mediation in Amorica at that time, this visit seems unlikely.