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What is the current Academic consensus - Anglo-Saxon Invasion, Anglo-Saxon Migration or none of the above? by /u/alriclofgar

The evidence is quite complicated, and is in fact the subject of an enormous debate. There are several key positions that most modern scholars oscillate between. Presumably you're referring to the recent book by Susan Oosthuizen on 'The Emergence of the English', which argues that there is very little reason to assume the Saxon mercenaries you describe having stayed, and which draws upon recent critical approaches to the archaeology of migration to underline this further. Oosthuizen's is a defensible but somewhat radical departure from current consensus. The debate is quite complicated so it may help to outline it a bit here.

Most scholars are in agreement over there being some form of migration having taken place, most of the dispute, concerns its nature, scale, and impact. The majority of scholars now align toward what usually gets called the 'elite replacement' model, which entails the idea that after a small scale migration and political takeover like that you describe, the Romano-British population, or at least, those parts of it visibly using elite material culture which survives to us in funerary remains and what little we can detect of settlement archaeology, assimilated to the culture of the new arrivals. Most scholars tend to assume this process of assimilation happened over the course of several centuries. Examples of texts arguing for this might be N.J. Higham's, Rome, Britain and the Anglo-Saxons.

Where the debate gets quite complicated owes to the complicated nature of ethnic identity, which scholars now largely regard as a situational construct (i.e., it is imagined, but not imaginary). Some scholars have pointed out that this has implications for our ability to detect things like ethnic identity using material culture alone, i.e., it's something that exists inside people's heads, and we can't prove it's there using the pots, belt buckles, swords, etc, that they buried with their dead alone. That sounds quite counter-intuitive (after all, if someone is burying their dead in a way that looks the same as the place the Saxons came from, and using the same kind of house, doesn't that mean they're probably a Saxon too?), but it relies on sociologists who've studied ethnic identity and found, even in the modern world with stable nation-states, ethnic identity very rarely conforms to our common sense assumptions about it (e.g. Rogers Brubaker, Ethnicity without Groups). This gets even tougher in the late fourth/early fifth century and beyond, where our only sources up to and including Gildas (you can read more detail about him in the replies made by Alric in the link posted in reply to this) are Roman authors, representing barbarians who exist outside of the Roman world system and who are depicted in stereotyped ways. We don't know if the diverse groups who migrated to Britain from northern Germania all directly associated with each other as ethnic compatriots all the time. Some work has even suggested, due to the Empire's influence in the region and its regular recruitment of soldiers from Germania in the Roman army, that they might have thought of themselves first and foremost as Roman soldiers (for arguments outlining all of these points check out Guy Halsall, Barbarian Migrations and the Roman West, Guy Halsall, Worlds of Arthur, James Harland, 'Rethinking Ethnicity and "Otherness" in Early Anglo-Saxon England, James Harland, "Memories of Migration? The 'Anglo-Saxon' Burial Costume of the Fifth Century AD.")

What does that mean then? Well, it means that some scholars have used this material to suggest a migration didn't happen, because we can't associate this material culture with the ethnic identity of its users, and thus can't directly trace migrating Saxons across a map in the same way that scholars like E.T. Leeds might have done in the early Twentieth Century in order to create rather neat maps for political history (Oosthuizen is the best example of this). They suggest we should look to longstanding communication and trade mechanisms shared between Britain and the North Sea world instead.

I'm of the opinion that the volume of material which moved alongside those sources you describe makes it difficult to discount that there took place some sort of migration event that changed how socio-political identification looked, prompted initially by the recruitment of Saxon soldiers and then the communication networks they forged lasting over several centuries, but one can certainly understand how the difficulties with approaching the source materials I've just outlined can lead to more extreme anti-migrationist positions. Even arguing for the occurrence of migration, the arguments about not assuming this means we can trace ethnic affiliation in the archaeological record remain important (and, importantly in my view, do not argue against a migration, simply claim that we can't know the socio-political identification of those who migrated). Collapses of Empires are messy affairs which involve the reconfiguration of how people identify, and any neat narrative which provides binaries of 'mass migration versus no migration', with images of clear sides, allegiances, and neat maps with lines moving across them will ultimately prove dissatisfying and misrepresentative.