Saxons along with Angles, Frisians and Jutes, invaded or migrated to the island of Great Britain (Britannia) around the time of the collapse of Roman authority in the west. Saxon raiders had been harassing the eastern and southern shores of Britannia for centuries before, prompting the construction of a string of coastal forts called the Litora Saxonica or Saxon Shore, and many Saxons and other folk had been permitted to settle in these areas as farmers long before the end of Roman rule in Britannia.
According to tradition, the Saxons (and other tribes) first entered Britain en masse as part of a deal to protect the Britons from the incursions of the Picts, Gaels and others. The story as reported in such sources as the Historia Brittonum and Gildas indicates that the British king Vortigern allowed the Germanic warlords, later named as Hengist and Horsa by Bede, to settle their people on the Isle of Thanet in exchange for their service as mercenaries. Hengist, according to Bede, manipulated Vortigern into granting more land and allowing for more settlers to come in, paving the way for the Germanic settlement of Britain.
Historians are divided about what followed: some argue that the takeover of southern Great Britain by the Anglo-Saxons was peaceful. There is, however, only one known account from a native Briton who lived at this time in the mid-5th century AD, (Gildas), and his description is of a forced takeover:
For the fire…spread from sea to sea, fed by the hands of our foes in the east, and did not cease, until, destroying the neighbouring towns and lands, it reached the other side of the island, and dipped its red and savage tongue in the western ocean. In these assaults…all the columns were levelled with the ground by the frequent strokes of the battering-ram, all the husbandmen routed, together with their bishops, priests, and people, whilst the sword gleamed, and the flames crackled around them on every side. Lamentable to behold, in the midst of the streets lay the tops of lofty towers, tumbled to the ground, stones of high walls, holy altars, fragments of human bodies, covered with livid clots of coagulated blood, looking as if they had been squeezed together in a press; and with no chance of being buried, save in the ruins of the houses, or in the ravening bellies of wild beasts and birds; with reverence be it spoken for their blessed souls, if, indeed, there were many found who were carried, at that time, into the high heaven by the holy angels… Some, therefore, of the miserable remnant, being taken in the mountains, were murdered in great numbers; others, constrained by famine, came and yielded themselves to be slaves for ever to their foes, running the risk of being instantly slain, which truly was the greatest favour that could be offered them: some others passed beyond the seas with loud lamentations instead of the voice of exhortation…Others, committing the safeguard of their lives, which were in continual jeopardy, to the mountains, precipices, thickly wooded forests, and to the rocks of the seas (albeit with trembling hearts), remained still in their country.
The Anglo-Saxon Chronicle, commissioned by King Alfred the Great, drew on earlier oral traditions and on the few written fragments available. The best of these, written around 730, was by the monk Bede whose history of English Christianity had the following brief account of the origin and distribution of the Angles:
“…from the Angles, that is, the country which is called Anglia, and which is said, from that time, to remain desert to this day, between the provinces of the Jutes and the Saxons, are descended the East Angles, the Midland Angles, Mercians, all the race of the Northumbrians, that is, of those nations that dwell on the north side of the river Humber, and the other nations of the English.”
(Bede’s Ecclesiastical History of the English Nation, Book I, Chapter XV, 731 A.D.)
The phrase “north of the Humber” refers to the northern kingdom of Northumbria, which includes what is now north and north-eastern England and part of southern Scotland. Mercia was located in central England and broadly corresponds to what is now known as the English Midlands.
This account can be related to the evidence of archaeology, notably the distribution of types of fibulae, or brooches, worn by the women. In essence, there are two kinds at issue, the saucer brooch and the cruciform brooch. East coastal and northern Britain were settled by women wearing cruciform brooches, which were in use in coastal Scandinavia, all of Denmark, and Schleswig-Holstein all the way south to the lower Elbe and all the way east to the Oder, as well as a pocket in coastal Friesland.
Southern England, excepting Kent was settled by women wearing the saucer brooch, which came from Lower Saxony, the south side of the lower Elbe, and pockets in the lands of the Franks up the Rhine and along the coast to the mouth of the Seine. These are the areas of England that are labeled explicitly as Saxon: Sussex, Wessex, and Essex. The settlement of Kent is attributed to Jutes, who originated in the land to the north of Angeln.
A more complete presentation is given under Angles.
Gildas then describes how the Saxons were slaughtered at the battle of Mons Badonicus forty four years before he writes his history, and Britain reverts to Romano-British rule. The 8th-century English historian Bede disagrees with Gildas, and states that the Saxon invasions continued after the battle of Mons Badonicus, including also Jutish and Anglic expeditions, resulting in a swift overrunning of the entirety of South-Eastern Britain, and the foundation of the Anglo-Saxon kingdoms.
Four separate Saxon realms emerged:
East Saxons: created the Kingdom of Essex.
Middle Saxons: created the province of Middlesex
South Saxons: led by Aelle, created the Kingdom of Sussex
West Saxons: created the Kingdom of Wessex
During the period of the reigns from Egbert to Alfred the Great, the kings of Wessex emerged as Bretwalda, unifying the country and eventually forging it into the kingdom of England in the face of Viking invasions.