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The Roman departure from Britain was completed by 410. The archaeological records of the final decades of Roman rule show undeniable signs of decay. Urban and villa life had grown less intense by the fourth quarter of the fourth century, pottery shards are not present in levels dating past 400, and coins minted past 402 are rare. So when Constantine III was declared Emperor by his troops in 407, and crossed the channel with the remaining units of the British garrison, effectively Roman Britain ended. The inhabitants were forced to look to their own defences and government -- a fact made clear in a rescript the emperor Flavius Augustus Honorius sent them in 410.

The remainder of this article covers the story of the Roman departure as told by Geoffrey of Monmouth's Historia Regum Britanniae, a dubious and untrustworthy historical document.

Geoffrey of Monmouth writes in his Historia Regum Britanniae that the final call for help from Britain came following the assassination of Gracianus Municeps. As soon as he was killed, forces building in Ireland under the command of Wanius and Melga, the kings of the Picts and Huns respectively, invaded once again, this bringing Scots, Norwegians, and Danes. Britain, stripped of all able-bodied men and women due to the adventurer Magnus Maximus's campaigns in Germany and Rome (383–388), called to Rome for help. Rome responded with a legion of troops who swiftly destroyed the invaders' armies causing them to flee once more.

Once freed of the threat, Rome constructed one last wall between Albany and Deira but they required the Britons to help construct it and maintain it. After the wall was completed, the Romans announced their intent to leave the island once and for all. All the men of Britain were sent to Londinium to be trained in the ways of combat to receive vast resources for building war machines and fortifications against attacks. Following that, the Romans legions left Britain never to return again.

Immediately upon hearing of Rome's departure, the enemy kings attacked for a third time and seized all the land down to the newly-constructed wall, causing the Britons to flee. Cities were sacked and entire villages emptied or murdered. The Britons pleaded for aide from Rome once more, but Rome had abandoned Britain to the ravages of the invaders, so they asked Guithelinus, the Archbishop of London, to seek help from their Briton cousins in Brittany. Guithelinus went to Gaul and begged for help from Aldroenus, king of Brittany, who granted his request and sent his brother Constantine with two thousand soldiers to help save Britain from the invaders. Constantine fought against the invaders and rallied the Britons behind him. After defeating them he was crowned Constantine II of Britain (this same individual is also called Constantine III of Rome).

According to Geoffrey's account, Constantine was murdered by a servant in the employ of Vortigern, who had designs on the throne. In reality, Constantine's troops declared him Western Roman Emperor in 407, and he took his armies to the continent to secure the claim. Britain was left defenseless, and Constantine was eventually killed in battle.

• Roman Britain

• The History Files The End of Roman Britain