Traditionally the remainder of the army of Maximus, after his defeat and execution by Theodosius in A.D.388, settled in Amorica which became known as "Lesser Britain", Brittany.
One ancient source for this is the ninth century Historia Brittonum gives the following account of Maximus and assigns him an important role:
"The seventh emperor was Maximianus. He withdrew from Britain with all its military force, slew Gratianus the king of the Romans, and obtained the sovereignty of all Europe. Unwilling to send back his warlike companions to their wives, families, and possessions in Britain, he conferred upon them numerous districts from the lake on the summit of Mons lovis, to the city called Cant Guic, and to the western Tumulus, that is Cruc Occident. These are the Armoric Britons, and they remain there to the present day. In consequence of their absence, Britain being overcome by foreign nations, the lawful heirs were cast out, till God interposed with his assistance."(2)
On 31 December in 406 several tribes of Barbarian invaders, including the Vandals, the Burgundians, the Alans and the Sueves, crossed the Rhine perhaps near Mainz, and overran the Roman defensive works in a successful invasion of the Western Roman Empire. This was a blow to the Western Empire from which it never recovered. The Roman authorities were never able to eject or destroy these invaders, most of whom eventually settled in Spain and North Africa, nor to face the movements of the Franks, Burgundians and Visigoths in Gaul at the same time.
Also, a contributing factor of major importance was the disunity among the Romans themselves. A unified Empire with the full support of a loyal population willing to make the necessary sacrifices to overcome invaders/settlers had shown in the past it was possible to keep the Empire's borders secure.
At the time of this invasion, the provinces of Britain were in revolt, setting up and pulling down a series of usurpers, which ended with the elevation of Constantine (III) early in 407. Fearful of a Germanic invasion and desperate for some sense of security in a world rapidly falling apart, the Roman military in Britain chose as their leader a man named after the famed emperor of the early fourth century, Constantine the Great, who had himself risen to power through a military coup in Britain. A common soldier, but one of some ability, Constantine moved quickly. He crossed the English Channel to the continent at Bononia (Boulogne) and (historians have assumed) took along with him all of the mobile troops left in Britain, thus denuding the province of any first line military protection and explaining their disappearance in the early fifth century.
About the same time Saxon pirates raided Britain, which Constantine had left defenseless. Obviously upset that Constantine had neglected them in his efforts to establish his own empire and had failed to defend them against the assaults they had hoped he would prevent, the Roman inhabitants of Britain and Armorica rebelled against Constantine's authority and expelled his officials. (3)
On 24th. August 410 the gates of Rome were opened from inside (by plebian malcontents?) and Alaric the king of the Visigoths entered the Imperial city.
Eventually the Roman general Constantius imprisoned Constantine and had him beheaded in 411.
Roman rule never returned to Britain after the death of Constantine III: as the historian Procopius later explained, "from that time onwards it remained under [the rule] of tyrants."
(1) For the Welsh literary source see:-
http://en.wikisource.org/wiki/The_Mabinogion/The_Dream_of_Maxen_Wledig
(2) See http://en.wikipedia.org/wiki/Historia_Brittonum
(3) There is a good summary of chronological events in the early Dark Ages in:-
"The quest for Arthur's Britain" edited by Geoffrey Ashe and published by Paladin.
