Bede's Ecclesiastical History of England, ed. by A.M. Sellar, , at sacred-texts.com
IN the year of our Lord 407, Honorius, the younger son of Theodosius, and the forty-fourth from Augustus, being emperor, two years before the invasion of Rome by Alaric, king of the Goths, when the nations of the Alani, Suevi, Vandals, and many others with them, having defeated the Franks and passed the Rhine, ravaged all Gaul, Gratianus, a citizen of the country, was set up as tyrant in Britain and killed. In his place, Constantine, one of the meanest soldiers, only for the hope afforded by his name, and without any worth to recommend him, was chosen emperor. As soon as he had taken upon him the command, he crossed over into Gaul, where being often imposed upon by the barbarians with untrustworthy treaties, he did more harm than good to the Commonwealth. Whereupon Count Constantius, by the command of Honorius, marching into Gaul with an army, besieged him in the city of Arles, took him prisoner, and put him to death. His son Constans, a monk, whom he had created Caesar, was also put to death by his own follower Count Gerontius, at Vienne.
Rome was taken by the Goths, in the year from its foundation, 1164. Then the Romans ceased to rule in Britain, almost 470 years after Caius Julius Caesar came to the island. They dwelt within the rampart, which, as we have mentioned, Severus made across the island, on the south side of it, as the cities, watch-towers, bridges, and paved roads there made testify to this day; but they had a right of dominion over the farther parts of Britain, as also over the islands that
are beyond Britain.