On December 31, 406, several tribes of Germanic invaders including the Vandals, the Burgundians, the Alans and the Sueves, crossed the frozen Rhine river near Mainz, and overrun the Roman defensives works in a successful invasion of the Western Roman Empire. This was a mortal blow to the empire from which it never recovered.
At the time of this invasion, the provinces of Britain were in revolt, setting up and pulling down a series of emperors, which ended with the elevation of Constantine early in 407. A common soldier, but one of some ability, Constantine moved quickly. Constantine crossed over the English Channel to the continent at Bononia, and historians have assumed he took with him all of the mobile troops left in Britain, thus denuding the province of any military protection and explaining their disappearance in the early fifth century. Constantine's two generals Justinianus and the Frank Nebiogastes, leading the vanguard of his forces, were defeated by Sarus, Stilicho's lieutenant, with Nebiogastes being first trapped in, then killed outside of, Valence. However, Constantine sent forth another army headed by Edobich and Gerontius, and Sarus was forced to retreat into Italy, needing to buy his passage through the Alpine passes from the Bacaudae who controlled them. Constantine secured the Rhine frontier, and garrisoned the passes that led from Gaul into Italy. By May, 408 he had made Arles his capital, where he appointed Apollinaris, the grandfather of Sidonius Apollinaris, as prefect.
In the summer of 408, as the Roman forces in Italy assembled to counterattack Constantine, Constantine had other plans. Fearful that several cousins of the emperor Honorius in Spain, which was a stronghold of the House of Theodosius and loyal to the ineffectual emperor, would organize an attack from that direction while troops under Sarus and Stilicho attacked him from Italy in a pincer maneuver, he struck first at Spain. He summoned his eldest son Constans from the monastery where he was dwelling, elevated him to Caesar, or assistant Emperor, and sent him with the general, Gerontius, towards Spain. The cousins of the throne were defeated without much difficulty and two -- Didymus and Theodosiolus -- were captured, while two others -- Lagodius and Verianus -- managed to escape to safety in Constantinople.
Constans left his wife and household at Caesaraugusta under the care of Gerontius to return to report to Arles. Meanwhile the Roman army mutinied at Ticinum on August 13, which was followed by the execution of the patrician Stilicho on August 22. As a by-product of these events, the actions of an intrigue within the Imperial court, the general Sarus abandoned the Imperial army followed by his men, leaving the emperor Honorius in Ravenna without any significant military power, who also faced the problem of a Gothic army under Alaric roaming unchecked in Etruria. So when Constantine's envoys arrived to parlay at Ravenna, Honorius eagerly recognized Constantine as co-emperor, and the two were joint consuls for the year 409.
That year was the high-water mark of Constantine's success. By September, the barbarians who had penetrated the Rhine defenses, and had spent the intervening two years roaming and plundering their way through Gaul, had reached the Pyrenees, where they broke through Constantine's garrisons and entered Spain. While Constantine prepared to send his son Constans back to deal with this crisis, word came that his general Gerontius had rebelled, raising his own man as co-emperor. Despite Constantine's best efforts, his fear of an attack from Spain did come to pass in the following year, when Gerontius advanced with the support of barbarian allies.
About the same time, Saxon pirates raided Britain which Constantine had left defenseless. Obviously upset that Constantine could no longer effectively defend them, the Roman inhabitants of Britain and Armorica rebelled and expelled his officials.
Constantine's response to this tightening circle of enemies was a final desperate gamble: with the remaining troops left to him, he marched on Italy, encouraged by the entreaties of one Allobich, who wanted to replace Honorius with a more capable ruler. But this invasion ended in defeat, with Allobich losing his life, and Constantine was forced to retreat into Gaul in the late spring of 410. Constantine's position grew even more desperate. His forces facing the rebel Gerontius were defeated at Vienne (411), where his son Constans was captured and executed. Constantine's Praetorian prefect Decimius Rusticus, who had replaced Apollinaris a few years earlier, abandoned Constantine, to be caught up in the rebellion of Jovinus in the Rhineland. Gerontius trapped Constantine inside of Arles, and besieged him.
At the same time a new general was found to support Honorius, the future Constantius III, who arrived at Arles, put Gerontius to flight, then took over the siege of Constantine in Arles. Constantine held out, hoping for the return of his general Edobich, who was raising troops in northern Gaul amongst the Franks, but on his return Edobich was defeated with a simple stratagem. Constantine's last slender hope faded when his troops guarding the Rhine abandoned him to support Jovinus, and he was forced to surrender. Despite his promise of safe passage, and Constantine's assumption of clerical offices, Constantius imprisoned the former soldier and had him beheaded in either August or September of 411.
Although Gerontius committed suicide in Spain, and Constantius later suppressed the revolt of Jovinus, Roman rule never returned to Britain after the death of Constantine III: as Procopius later explains, "from that time it remained under [the rule] of tyrants."
The primary sources are discussed by C.E. Stevens, "Marcus, Gratian, Constantine", Athenaeum, 35 (1957), 316-47, and E.A. Thompson, "Britain, A.D. 406-410", Britannia, 8 (1977), 303-318.