A good article written by Richard Denning on the origins of the Anglo-Saxons:
From AD 70 until around AD 400 most of Britain was ruled from Rome as the Province of Britannia. As pressures from outside the empire, and weaknesses and corruption from inside grew Rome abandoned Britain. The result was that the Romano-British (who would one day become the Welsh) were left to defend themselves from Picts and Irish and Germanic Raiders.
The Anglo-Saxon Chronicle and other records suggest that after Rome officially told the former province of Britannia to look to its own defences in about 416 AD that the British invited in Germanic mercenaries to help defend Britain. Probably German tribes were already migrating to Britain in the Roman period as the Romans used Germans in their Legions but in 449 there is this entry. The British King Vortigern invited Hengest and Horsa to come to Britain.
Hengest and Horsa were followed by many other warlords and their followers. At first they fought for the British and settled peacefully along the eastern and Southern coasts. Eventually several things may have occurred…
Read the entire article.
In conjunction with Richard’s article, I wrote a post about the Anglo-Saxons a few years back.