United Kingdom

The reforms of Diocletian ended the chaos of the 3rd century and ushered in the late imperial period. Britain, however, for a short time became a separate empire through the rebellion (286/287) of Carausius. This man had been in command against the Saxon pirates in the Channel and by his naval power was able to maintain his independence. His main achievement was to complete the new system of Saxon Shore forts around the southeastern coasts. At first he sought recognition as coemperor, but this was refused. In 293 the fall of Boulogne to Roman forces led to his murder and the accession of Allectus, who, however, fell in his turn when Constantius I invaded Britain in 296. Allectus had withdrawn troops from the north to oppose the landing, and Hadrian’s Wall seems to have been attacked, for Constantius had to restore the frontier as well as reform the administration. He divided Britain into four provinces, and in the same period the civil power was separated from the military. Late Roman sources show three separate commands respectively under the dux Britanniarum (commander of the Britains), the comes litoris Saxonici (count of the Saxon Shore), and the comes Britanniarum, though the dates of their establishment are unknown and may not have been identical.

The 4th century was a period of great prosperity in towns and countryside alike. Britain had escaped the barbarian invasions of the 3rd century and may have seemed a safe refuge for wealthy continentals. Its weakness lay in the fact that its defense was ultimately controlled by distant rather than local rulers. The garrison was perhaps weakened by withdrawals for the civil war of Magnentius (350–351); at any rate in 367 a military disaster occurred due to concerted seaborne attacks from the Picts of Scotland and the Scots of Ireland. But, though the frontier and forts behind it suffered severely, there is little trace of damage to towns or villas. Count Theodosius in 369 restored order and strengthened the defenses of the towns with external towers designed to mount artillery. Prosperity continued, but the withdrawals of troops by Magnus Maximus in 383 and again at the end of the century by Stilicho weakened security. Thus, when Constantine III, who was declared emperor by the army in Britain in 407, took further troops to Gaul, the forces remaining in the island were insufficient to provide protection against increasing Pictish and Saxon raids. The Britons appealed to the legitimate emperor, Honorius, who was unable to send assistance but authorized the cities to provide for their own defense (410). This marks the end of Roman Britain, for the central government never reestablished control, but for a generation there was little other outward change.

Power fell gradually into the hands of tyrants. Chief of these was Vortigern (c. 425), who, unlike earlier usurpers, made no attempt to become Roman emperor but was content with power in Britain. Independence was producing separate interests. By this date Christianity had made considerable headway in the island, but the leaders followed the heretical teaching of Pelagius, himself a Briton, who had emphasized the importance of the human will over divine grace in the achievement of salvation. It has been held that the self-reliance shown in the maintenance of national independence was inspired by this philosophy. Yet there was also a powerful Roman Catholic party anxious to reforge the links with Rome, in support of whom St. Germanus of Auxerre visited Britain in 429. It may have been partly to thwart the plans of this party that Vortigern made the mistake (c. 430; the date given by the Anglo-Saxon Benedictine scholar Bede [d. 735] is between 446 and 454) of inviting Saxons to settle and garrison strategic areas of the east coast, though he certainly also had in mind the need to ward off seaborne raids by Picts, which at this time were troublesome.