The Fall of Roman Power in Britain started as early as 288-290AD, when Carausius declared himself Emperor in Britain and ruled as such until 294AD when he was assassinated by Allectus, who claimed the rule and was himself defeated by Emperor Constantius in 297AD, who brought an army to Britain and reconquered it, bringing back Rome's rule to Britain. Allectus had stripped all the garrisons for troops to resist Constantius, weakening the defenses of Britain.
Constantius repaired this weakened state, rebuilding the fortress at York, where he made his residence. He also had the fortifications along Hadrian's Wall repaired, stationing fresh garrisons in those fortifications. He created a series of forts along the Saxon Shore to deal with the Saxon raids. These forst extended from the Solent to the Wash and perhaps beyond to the Humber. These were large forts defended by strong masonry walls and mixed cohorts of cavalry and infantry. He placed these forts under the command of the Comes of the Saxon Shore.
When Constantius withdrew from Britain, the new fortifications and garrisons seemed adequate to defend the province. Peace was maintained until after the middle of the fourth century.
Then in 367AD disaster struck, with the Great Invasion. Picts came pouring across Hadrian's Wall in greater numbers than ever, and the Scotii of Ireland invaded across the Irish Sea, they were joined by Saxon invaders, overrunning garrisons and burning towns and villas throughout a large area of Roman Britain. The Walled Towns held out against the invaders, but most of the rural areas were plundered and the villas laid waste.
Theodosius was sent with his army to bring order to Britain. When he landed in Kent he found Barbarian raiding parties pillaging right up to the gates of Londinum. Theodosius cleared the invaders from the province and restored peace, after a forceful campain.
He restored the fortifications and remanned them. He also set up a system of signals to warn of invaders. But it was too late, the invasion of 367AD had done too much permanent damage to the province's economy to maintain the level of defense needed.
In 383AD the fate of Britain was sealed, when Magnus Maximus usurped the title of Emperor and took his legions out of Britain, leaving only an inadequate force to defend the province. He crossed into Gaul and marched on Rome. Maximus was defeated and killed in 388AD by Theodosius (the son of the Theodosius who earlier saved Britain).
A few years later, Britain was once again saved by the great Roman general Stilicho, who once again reorganized the defense of the province. After he returrned to Rome and Rome's wars, Britain was left once again to defend itself.
In 407AD Constantine, a usurper who had risen from the local Legions, once again drained Britain of troops and crossed the channel, never to return.
In 410AD Honorius declared that the British province must defend itself and provide its own troops. Britain was now on its own.
Through the next hundred and fifty years Roman Britain fought a losing battle against Barbarian invaders.
It is in this period we find the legends of Arthur (Artorius), Vortigern, Aurelius Ambrosianus, Hengist and Horsa.