When the Romans withdrew the legions from Britain and left the province weakened, three strong Germanic peoples began an invasion which would over time submerge the Romano British culture. These were the Saxons from northern Germany, the Angles from the southern area of the Jutland peninsula, the area known as Angeln, and the Jutes from Jutland Proper. The Roman chroniclers of the period seem to identify all of these peoples, Saxons, Angles, Jutes, and the Frisians as Saxons and/or Anglo-Saxons. They were not though at this time one people, that came later after the disappearance of the last lowland Roman-British states in the early seventh century.
The First Saxons to arrive in Britain came by invitation to serve as mercenaries in the defense of Britain against invading Irish tribes, the Scotii, and the Picts of Northern Britain. The largest of these early invasions of Saxon mercenaries were those lead by Hengist and Horsa some time between c.440-460AD. They were employed by Vortigern and set up their own petty kingdoms, and for the next two centuries fought to overcome the native Romano-Celtic people.
At times they were slowed and even driven back by such leaders as Ambrosius Aurelanius and Artorius. But eventually the Saxons were victorious and Roman Britain became Anglo-Saxon England.