Approximately 367 years had passed between Rome’s conquest of Britannia (in 43 A.D.) and its departure from the island (in 410). By way of comparison, in the presidential-election year of 2008, a mere 232 years had passed since America’s founding fathers signed the Declaration of Independence.
One can only imagine the power vacuum which existed in Britannia after Rome’s legions left. Who would rule? Whose forces would defend the people and their land? Rome itself had experienced difficulty keeping out intruders along the northern border. First a road, then a wall, proved ineffective. What could native Britons do?
Ancient sources provide us with answers. Recall this is the time in history known as "The Dark Ages." That description applies since so few unbiased contemporary written sources remain (or were ever created).
Although we are "in the dark" about much, we do know from the monk Gildas (writing the polemic De Excidio Britanniae [The Ruin of Britain] in approximately 540 A.D.) and later from Venerable Bede (an 8th century monk living in Northumbria who completed his History of the English Church and People in 731), that Britons sought assistance from foreigners to help them resist intruders.
Before they sought help elsewhere, as Gildas notes, Britons had appealed to Rome - specifically to Aetius, the Roman commander in Gaul (today’s France) - for help. In approximately 427, the Britons’ letter pleaded with Aetius:
The barbarians push us back to the sea, the sea pushes us back to the barbarians; between these two we are either drowned or slaughtered.
When Aetius did not provide requested assistance, the Britons invited Angles, Saxons and Jutes to help. Those invited foreigners, however, would soon become invading intruders.