Beowulf

As Britons tried to resist their former defenders, warring factions pushed the country into deepening chaos. The Gallic Chronicles of 511 depict a desperate situation in the year 440:

The Britains, lost to the Romans, yield to the power of the Saxons.

By the following year (441), the people were nearing capitulation:

The Britains, which to this time had suffered from various disasters and misfortunes, are reduced to the power of the Saxons. (Gallic Chronicles of 452)

Gildas paints a scene of anarchy by 455 (or thereabouts):

Then they [the Saxons] again complained that their monthly allowance was insufficient, purposely giving a false colour to individual incidents, and swore that they would break their agreement and plunder the whole island unless more lavish payments were heaped on them. There was no delay.

As the situation grew more and more hopeless for the Britons, Gildas relates actions of total despair by the mid-fifth century:

So a number of the wretched survivors were caught in the mountains and butchered wholesale. Others, their spirit broken by hunger, went to surrender to the enemy; they were fated to be slaves forever, if indeed they were not killed straight away, the highest boon. Others made for lands beyond the sea. Others held out, though not without fear, in their own land, trusting their lives with constant foreboding to the high hills, steep, menacing and fortified, to the densest forest and to the cliffs of the sea coast.

In other words, native Britons were being killed, displaced, enslaved or segregated by fearsome invaders who would ultimately change the culture of the country. Over time, even the language was different.