Constantine III, whose actual name was Flavius Claudius Constantinus, was in 407 illegaly appointed emperor in Britain by the soldiers themselves. Constantine crossed over into Gaul and ruled there until 411, when he was killed.
Constatine III and his son Constans were partial sources for King Constantine of Britain and his son King Constans in the pseudo-historical Arthurian chronicles, and for King Constant and his son King Monk in the Story of Merlin.
True History of Constantine[]
In 405 or 406 tribes of Germanic invaders, including the Vandals, the Burgundians, the Alans and the Sueves, crossed the Rhine. The emperor Honorius was unable to stop them.
Meanwhile, perhaps in response to this crisis and lack of imperial authority, the troops in Britain selected a certain Marcus and appointed him emperor. Marcus was soon killed by his own troops. Then, late in 406, they made an emperor of another one of their own, a man named Gratian. But after a four month reign, he also was killed by his own troops. Then, in 407, the troops tried again, with a man named Constantine.
Constantine crossed over to Gaul and eventually drove Sarus, a Roman General appointed by the supposed true emperor Honorius, back into Italy.
Constantine ruled Gaul (and Britain) from Arles. Constantine’s eldest son Constans, who had been made a monk, left the monastery to join his father. Constantine gave to his son the title Caesar, effectively making him co-emperor and then sent him with troops into Spain to fight the relatives of the emperor Honorius, Didymus and Verinian. Constans was victorious.
Meanwhile, on Honorius’ side, the Roman army revolted. The general Stilicho was killed, and Sarus and his men deserted. Accordingly Honorius was forced to recognize Constantine as co-emperor in the west, ruling over Gaul, Spain, and Britain. Honorius and Constantine were joint consuls for the year 409.
But in September of that year, the raiding Vandals, Burgundians, Alans and, Suebi broke though the guard which Constans had left on the Pyranees and entered Spain. The Suebi later destroyed the Alans, but their descendants still live in northen Spain. The Vandals went on to Africa, gave their name to Andelusia, and founded a stable kingdom of their own.
Gerontius, whom Constans had left in Spain, now revolted and installed his friend Maximus as emperor at Tarragonia. Then Gerontius began to stir up Gaul. The Armoricans revolted against Constantine.
In 410 the Britons threw out Constantine’s officials and proclaimed loyalty to Honorius. But Honorius could do no more than tell the Britons to take charge of their own defense, effectively removing Britain from the Roman empire, unless this note by Zosimus should in fact refer to Brutium in Italy, not the province of Britain.
Constantine now only controlled southern Gaul. Constans tried to reconquer Spain, but was defeated by Gerontius who capturef Constans at Vienne and put him to death. Gerontius then marched against Constantine, but instead met a Roman army faithful to Honorius. Most of Gerontius’ troops deserted him. Those who stuck with him then revolted, and Gerontius and his wife Nonnichia ended by commiting suicide.
Constatine was now besieged in Arles by Honorius’ troops led by a general named Constantius. Constantine, seeing that all was lost, had himself ordained as a priest in the Church, the intention being that Honorius would see that he had nothing to fear from a priest. Constantine then surrendered to Honorius’ son Julian. But on the journey to Rome, they were waylaid by brigands, and Constantine was beheaded.
The Account of Bede[]
Bede writes in Chapterr 11 of his “Ecclesiastical History” (as tranlated by L. C. Jane):
IN the year 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 Municeps was set up as tyrant and killed. In his place, Constantine, one of the meanest soldiers, only for his name’s sake, and without any worth to recommend him, was chosen emperor. As soon as he had taken upon him the command, he passed over into France, where being often imposed upon by the barbarians with faithless treaties, he caused much injury to the Commonwealth. Whereupon Count Constantius by the command of Honorius, marching into Gaul with an army, besieged him in the City of Arles, and put him to death. His son Constans, whom of a monk he had created Caesar, was also put to death by his own Count Gerontius, at Vienne.
In Geoffrey of Monmouth and later Pseudo-Historical Texts[]
Geoffrey of Monmouth in his Historia Regum Britanniae, tells how the Roman troops appointed Gracian as emperor, and of his death.
But then he makes no mention of a man named Constantine being appointed emperor. Instead Geoffrey inserts material from Gildas’ De Excidio et Conquestu Britanniae and from the Anglo-Saxon Chronicle telling how the Britons asked for help from Rome against the barbarians, how help was sent, and how a wall was built in the north. But as soon as the Romans again departed, Britain was yet again ovverun by barbarians: Scots, Picts, Norwegians, and Danes. Letters requesting help are sent to the Roman general Agicius. The time period is therefore 433–454 if Agicius is to be identified with Aëtius, and around 450–464 if Agicius is to be identified with Aegidius. In either case Geoffrey’s account has long gone beyond the death of Constantine III in 411.
The people of Britain then choose Constantine, the brother of King Aldroen of Little Britain, as their king.
This King Constantine, brother of Aldroen, is to some degree a replacement for the historical Constantine III. Like Constantine III, he has as his son a monk named Constans who leaves the monastery to become his heir. That at least is borrowed from the historical Constantine III.
That Constans in Geoffrey’s account has his own reign, following the death of Constantine, might come from an interpretation of Bede or some similar source which did not make it clear that Constans predeceased his father.
But Geoffrey’s Constantine differs from Constantine III in being a thoroughly domestic ruler, never leaving the country as far as we are told, while Constantine III left Britain very soon after being made ruler, and never returned.
Either some previous writer has very much garbled the true career of Constantine III, or only some features of Constantine III have been applied to another Constantine who was originally quite separate. See Custennin Gorneu, who is fifth in descent from Cynan in medieval genealogies, while Aldroen, the brother of Geoffrey’s Constantine, is said to be the fourth king after Conan.