An extract on #ig_sharepoint
Towards the end of the 4th century Britain came under increasing pressure from barbarian attacks, and there were not enough troops to mount an effective defence. After elevating two disappointing usurpers, the army chose a soldier, Constantine III, to become emperor in 407. He crossed to Gaul but was defeated by Honorius; it is unclear how many troops remained or ever returned, or whether a commander-in-chief in Britain was ever reappointed. A Saxon incursion in 408 was apparently repelled by the Britons, and in 409 Zosimus records that the natives expelled the Roman civilian administration. However, Zosimus may be referring to the Bacaudic rebellion of the Breton inhabitants of Armorica since he describes how, in the aftermath of the revolt, all of Armorica and the rest of Gaul followed the example of the Brettaniai. A letter from Emperor Honorius in 410 has traditionally been seen as rejecting a British appeal for help, but it may have been addressed to Bruttium or Bologna. With the imperial layers of the military and civil government gone, administration and justice fell to municipal authorities, and local warlords gradually emerged all over Britain, still utilizing Romano-British ideals and conventions. Laycock has investigated this process and emphasised elements of continuity from the British tribes in the pre-Roman and Roman periods, through to the native post-Roman kingdoms.
In British/Welsh tradition, pagan Saxons were invited by Vortigern to assist in fighting the Picts and Irish, though Germanic migration into Roman Britannia may have begun much earlier. There is recorded evidence, for example, of Germanic auxiliaries supporting the legions in Britain in the 1st and 2nd centuries. The new arrivals rebelled, plunging the country into a series of wars that eventually led to the Saxon occupation of Lowland Britain by 600. Around this time many Britons fled to Brittany (hence its name), Galicia and probably Ireland. A significant date in sub-Roman Britain is the Groans of the Britons, an unanswered appeal to Aetius, leading general of the western Empire, for assistance against Saxon invasion in 446. Another is the Battle of Deorham in 577, after which the significant cities of Bath, Cirencester and Gloucester fell and the Saxons reached the western sea.
Most scholars reject the historicity of the later legends of King Arthur, which seem to be set in this period, but some such as John Morris think there may be some truth to them.