Many years ago I stumbled across an article about an alternate theory about how the Anglo-Saxons came to Britain. The gist of it was that when studying the linguistics of British place names, there is a peculiar lack of Celtic-origin names in Eastern Britain and an overabundance of the same in Western Britain. The author derived from that the hypothesis that Britain was settled by Germanic and Celtic tribes from the same time as the Germanic settlement of mainland Europe, in roughly the same distribution as was found by William I.
The author further calculated that an invasion force as proclaimed by the more traditional theory was not feasible during the 4th-6th century CE, so what the chronicles described as an invasion was more likely an usurpation of power by resident Germanics / mercenaries. Lastly the author claimed that our traditional version of the Anglo-Saxon invasion is mostly derived from a single source, a Celtic priest based in Western Britain, who may have coloured his chronicles under the impression of a defeat in a local border war against his Germanic-British neighbours.
Unfortunately I cannot find the article anymore, it was rather elaborate with maps and lots of linguistic examples, I specifically remember the placename suffix "-ter" to be derived from "tree", which is an ancient Germanic word.
So, is there actually any serious debate about this type of theory? What is the accepted proof for a large-scale invasion of a densely populated country by bands of warriors from a sparsely populated region?
The invasion narrative presented by Gildas' de Excidio Britanniae and corroborated in Bede's Historia Ecclesiastica has, in fact, long been discredited. A revisionist theory, that only a tiny Germanic minority (or even no Anglo-Saxons at all!), has similarly been dismissed in favour of a more moderated approach.
The prevailing approach in modern discussions of the period is one based largely on the archaeological evidence of changing settlements across the fifth and sixth centuries, the material culture record, and, to an extent, pioneering but potentially-tenuous genome studies. The image that this research gives us is one of piecemeal settlement, colonisation and occasional conquest across the British Isles, with Germanic communities often settling alongside or even overlapping pre-existing communities, at times integrating into and at times assuming control over their surrounding areas. It should be noted that the arrival of the first wave of Angles and Saxons was preceded by a plague which purportedly devastated the British population, and evidence from sites such as Mucking in Essex suggest that in some cases, Germanic settlers were merely reoccupying land that had simply fallen out of use, or were working land on the fringes of extant communities before integrating into them over time.
It's likely that the English would have formed a military elite of sorts in many of the communities in which they arrived, and their integration could have been welcomed as a provision of stability in the face of the risk of raiding. It certainly appears that even leaders made attempts at integrating: The early sixth and seventh century kings of Kent, for example, identified at Cantwara Cyninges, claiming an identity aligned with the local British tribal polity rather than the Jutish Germanic identity which would be prevalent in Bede's ethno-nationalist conquest narrative.
The author derived from that the hypothesis that Britain was settled by Germanic and Celtic tribes from the same time as the Germanic settlement of mainland Europe.
If the author implied that Germanic settlers arrived in Britain during the Pre-Roman Iron Age, then that is unsupported.
If the author meant that Germanic settlers arrived at the very beginning of the Migration Period; then the sources to support this are scarce, but it is not impossible. Roman policy concerning Germanic tribes shifted after 378 CE, after which they structurally started to settle Germanic tribes as foederati within the empire. It is quite possible that some Germanic settlers were settled in Britain prior to the end of Roman rule in Britain in 410 CE.
What is the accepted proof for a large-scale invasion of a densely populated country by bands of warriors from a sparsely populated region?
Sub-Roman wasn't that densely populated and neither was the North Sea coast that sparsely populated.
During and just prior to the period of Germanic settlement, the British cities were affected by the plague of Justinian, Irish and Pictish raids (including the so-called Great Conspiracy) and a general dissipating state authority. A population of about 2 million people for the entirety of Britain below Hadrians Wall is generally taken as probable, with Southern and Eastern England likely containing about half of that population. This was not a unified populace, but one divided over many separate kingdoms and warlords.
At the same time, the coastal areas of the North Sea were affected by 5th century climate changes which seemed to have swamped significant parts and reduced the amount of arable land considerably; in effect creating overpopulation in certain areas.
The author further calculated that an invasion force as proclaimed by the more traditional theory was not feasible during the 4th-6th century CE, so what the chronicles described as an invasion was more likely an usurpation of power by resident Germanics / mercenaries.
This tends to be the current view.
The traditional / 19th century romantic view, such as put forward by figures like Edward Augustus Freeman, was one of noble but savage Anglo-Saxons arriving on the coasts of England in full force and subsequently subjugating and exterminating the inferior Welsh / Celtic populace of Roman Britain; who had lost their Roman protectors.
It has been almost completely depreciated.
The Anglo-Saxon invasion is generally thought to have actually been a continuous migration process that lasted several centuries. The first Germanic migrants were probably mercenaries hired by local British chiefs; not savage invaders. Based on archaeological evidence, the number of Germanic migrants is rather small. Around 200.000 people arriving within at least a 150-200 year period. Together with demographic models that show a marked increase in Anglo-Saxon population when only a slightly higher birth figure than the native-British is taken into account and the migration of Welsh/British populations to Brittany in modern France, it isn't difficult to imagine the Anglo-Saxon presence becoming more dominant.
As they (gradually) took over politically, their Germanic language was probably adopted by much of the remaining British populace. Language shift is a well-documented process and would have been promoted by the Anglo-Saxon legal system; which discriminated against non-Anglo-Saxons; but at the same time did not put up any barriers on becoming an Anglo-Saxon beyond speaking their Germanic language. Mind you, this probably wasn't a purposely designed strategy of cultural assimilation; but more likely shows how difficult it was to differentiate between the Welsh and Anglo-Saxons aside from the languages they spoke.
Lastly the author claimed that our traditional version of the Anglo-Saxon invasion is mostly derived from a single source, a Celtic priest based in Western Britain, who may have coloured his chronicles under the impression of a defeat in a local border war against his Germanic-British neighbours.
Probably Gildas or Bede.