Bede's Ecclesiastical History of England, ed. by A.M. Sellar, , at sacred-texts.com
In the year of our Lord 286, Diocletian, the thirty-third from Augustus, and chosen emperor by the army, reigned twenty years, and created Maximian, surnamed Herculius, his colleague in the empire. In their time, one Carausius, of very mean birth, but a man of great ability and energy, being appointed to guard the sea-coasts, then infested by the Franks and Saxons, acted more to the prejudice than to the advantage of the commonwealth, by not restoring to its owners any of the booty taken from the robbers, but keeping all to himself; thus giving rise to the suspicion that by intentional neglect he suffered the enemy to infest the frontiers. When, therefore, an order was sent by Maximian that he should be put to death, he took upon him the imperial purple, and possessed himself of Britain, and having most valiantly conquered and held it for the space of seven years, he was at length put to death by the treachery of his associate Allectus. The usurper, having thus got the island from Carausius, held it three years, and was then vanquished by Asclepiodotus, the captain of the Praetorian guards, who thus at the end of ten years restored Britain to the Roman empire.
Meanwhile, Diocletian in the east, and Maximian Herculius in the west, commanded the churches to be destroyed, and the Christians to be persecuted and slain. This persecution was the tenth since the reign of Nero, and was more lasting and cruel than almost any before it; for it was carried on incessantly for the space of ten years, with burning of churches, proscription of innocent persons, and the slaughter of martyrs. Finally, Britain also attained to the great glory of bearing faithful witness to God.