The coming of the ‘Dark Ages’ was predicated on the fall of Rome.  Rome had dominated Europe (and parts of Africa and Asia) for nearly 800 years when Alaric, a Visigoth, sacked it.  According to the excellent documentary, The Dark Ages (available on Netflix here), Alaric had served in the Imperial forces until passed over for a promotion, at which point, he took his cause directly to the City of Rome.  He and his men then camped outside the walls in 408 AD, cutting off all food and succor to the city, for two years, until in 410 the citizens opened the gates.  That Rome no longer had the military resources to relieve the city in all that time indicates the extent of its decline.

Read a great description of the 40-year lead-up to the sacking here:  http://www.mmdtkw.org/VAlaric.html

The story continues:  “Late in 407, Alaric again appears to have allied himself with Rome to participate in Stilicho’s projected expedition to the far borders of the Eastern Empire. Some modern historians interpret this as a Stilicho plan to get the Visigoths as far away as possible from northern Italy, but other events intervened. [Read about Stilicho here]

Everyone knew that “those horrible northern barbarians” had caused most of Rome’s troubles, and “anti-blond” racism and discrimination became rampant in the Western Empire. Wives and children of some Visigoth soldiers, officers, and noblemen, who until then had been faithful to Rome, were murdered in Italian-led pogroms. Visigoths fled en masse to Alaric’s protection, making Alaric’s forces all the stronger. Only Stilicho could save the day.

Stilicho, ever the realist, went to the Senate and asked for money to buy off Alaric. The Senate responded with anti-Visigoth polemics and personal attacks on Stilicho. Hadn’t he purposely allowed Alaric’s repeated escapes? And weren’t Stilicho and Alaric even now planning joint conquests in the east? Why was Stilicho trying to enrich Alaric? Wasn’t Stilicho half barbarian himself? (True.) Why should Rome pay tribute to blond, blue-eyed Barbarians — even if they had an overwhelmingly superior force?

And in Ravenna, Stilicho’s rivals in the Court of Honorius (now removed to the north) were busy convincing Honorius that Stilicho was plotting a coup that would put Stilicho’s own son, Eucherius, on the Imperial throne. Honorius, who had never been noted for his mental acuity, believed the story and signed a warrant for Stilicho’s arrest. It was served on Stilicho in a churchyard in Ravenna. With his troops far away and with popular sentiment turning against him, he took sanctuary in the church. He came out again after receiving assurances of a trial after which he would keep his head. But the local commander had lied and Stilicho was decapitated without trial on August 22, 408 AD.

So now we have a greatly expanded (and very angry) Visigoth army on the loose in Italy, and the only Roman general capable of opposing them is headless in Ravenna. The predictable result was that a few months later Alaric’s long siege of Rome began. The Senate now offered to buy off the Visigoths with gold, silver, and food, the last of which, of course, would have to be provided from outside Rome, probably by Honorius. Alaric withdrew for a while after receiving the Senate promises (and some gold and silver), but Honorius, well fed in Ravenna, was not susceptible to the pleas of the starving (some accounts say eventually cannibalistic) Romans. All deals fell through, and Alaric renewed the siege. Finally, on August 24, 410, someone opened the Salaria Gate (now the Pincian Gate at the end of Via Venetto), and Alaric’s Visigoths poured through.”  http://www.mmdtkw.org/VAlaric.html

The consequences for Britain were profound.  It is in 410 AD, in fact, that the Roman legions leave Britain undefended.  This is the entry for Gildas (a 6th century British cleric) and Zosimus (a Bytanzine historian from Constantinople, writing 491-518 AD):

410   Gildas

From Britain

envoys set out with their complaints … to beg help from the Romans. … The Romans … informed our country that they could not go on being bothered with such troublesome expeditions … for the sake of wandering thieves who had no taste for war. Rather, the Britons should stand alone, get used to arms, fight bravely, and defend with all their powers their land, property, wives, children, and, more importantly, their life and liberty.  …  they should not hold out to them for the chaining hands that held no arms, but hands equipped with shields, swords and lances, ready for the kill. This was the Romans’ advice.

Zosimus

When Alaric (the leader of the Visigoths) neither gained peace on the terms he proposed nor received any hostages, he again attacked Rome … and finally captured it. … Honorius sent letters to the cities of Britain, urging them to fend for themselves.

http://www.cit.griffith.edu.au/~s285238/DECB/DECBbestest.html

The following years in Britain were marked by near constant warfare, plague, and a flight from the cities.