與此同時,蠻族於408年襲擊了不列顛,但這些襲擊似乎都被擊退。於410年後,西羅馬皇帝霍諾留似乎寫信命令不列顛各城市需要自我保護。儘管這有時會引起爭議。[26][27][28]日耳曼劫掠者從5世紀中葉開始在不列顛東部河谷定居。[29] 其後內戰似乎已經爆發,這些內戰被後世學者解釋為親羅馬和獨立聚落之間,或“國教會”和伯拉糾主義黨派之間(Myres 1965年,Morris 1965年),農民和地主之間的階級鬥爭(Thompson 1977年,Wood 1984年),或是一場城市精英的政變(Snyder 1988年)。Stuart Laycock(英语:Stuart Laycock)在2008年的著作《不列顛尼亞失敗的國家Britannia the Failed State》探究一個新近的觀點認為不列顛根據各部落本身,猛烈地分裂成一個個不列顛人的王國; 猛烈地分裂是有爭議的,但顯然大多數不列顛尼亞羅馬公社(英语:civitates)逐漸轉變為王國。鄉郊似乎繼續著和以前一樣的生活,而歐塞爾的聖日耳曼努斯(英语:Germanus of Auxerre)造訪不列顛時描述城鎮的生活規模縮小。王權的長期鬥爭取代了過去羅馬行省中央管治的方式。
吉爾達斯認為軍閥沃蒂根召開了一個“議會”以尋找應對蠻族威脅的方法。議會按照羅馬的慣例選擇僱用撒克遜雇傭兵。一段時間過後,這些雇傭兵調轉槍頭反對不列顛人並劫掠城鎮。據說不列顛人領袖安布羅休斯·奧理安在很長一段時間內在許多戰役中與他們作戰。巴頓山戰役(英语:Battle of Badon)大約在490年這一時期將近結束時發生,後來的資料來源聲稱這場戰役是亞瑟王贏得的,儘管吉爾達斯沒有在著作中透露他的身份。此役之後,迎來一段漫長的和平時期。不列顛人似乎已經控制了英格蘭和威爾斯,邊界大致是從約克到般尼茅夫以西一帶。日耳曼的撒克遜人控制了從東約克郡到林肯郡,也許還有諾定咸郡,再到東盎格利亞和東南英格蘭一帶的弧形東部地區。
吉爾達斯大約在540年用拉丁文寫成描述了不列顛歷史的著作,但前半部分(其他可用資料來源)極度混亂。他嚴詞批評了不列顛西部的五位統治者的罪孽—頓諾尼亞的君士坦丁、奧勒留·卡尼努斯(Aurelius Caninus)、德梅泰(英语:Kingdom of Dyfed)的沃蒂波爾(Vortipor)、庫內格拉蘇斯(Cuneglasus)和馬格洛庫努斯(Maglocunus)(或稱為Mailcun或後來拼寫為格溫內斯的邁爾貢(英语:Maelgwn Gwynedd))。在著作中他還攻擊了不列顛的教士。他提供有關不列顛的飲食、服飾和娛樂的資訊。他寫道,布立吞人被殺、移居或被奴役,但沒有給出數字。
Esmonde-Cleary, A. S. (1989) The Ending of Roman Britain. London: Batsford
Fouracre, Paul (ed.) (2005) The New Cambridge Medieval History, Volume I, c.500–c.700. Cambridge: Cambridge University Press
Jones, Michael E. (1996) The End of Roman Britain Ithaca: Cornell University Press
Halsall, Guy (2013) Worlds of Arthur. Facts and Fictions of the Dark Ages. Oxford: Oxford University Press
Higham, Nicholas (1992) Rome, Britain and the Anglo-Saxons. London, Seaby
Higham, Nicholas (1994) The English Conquest: Gildas and Britain in the Fifth Century. Manchester University Press
Jones, Michael (1996) The End of Roman Britain. Ithaca: Cornell University Press
Lapidge, Michael & Dumville, David (1984) Gildas: New Approaches. Woodbridge: Boydell
Morris, John (1973) The Age of Arthur
Morris, John (1980) Nennius: British History and the Welsh Annals. Chichester: Phillimore
Morris, John (gen. ed.) Arthurian Period Sources volumes 1–9, general editor: John Morris, Phillimore & Co, Chichester (includes full text of Gildas & Nennius, St Patrick material and various annals and charters)
^Halsall, Guy Barbarian migrations and the Roman West, 376–568 Cambridge University Press; illustrated edition (20 Dec 2007) ISBN978-0-521-43491-1 pp.217–218
^Discussion in Martin Millett, The Romanization of Britain, (Cambridge: Cambridge University Press, 1990) and in Philip Bartholomew 'Fifth-Century Facts' Britannia vol. 13, 1982 p. 260
^Michael Jones(英语:Michael Jones) andJohn Casey(英语:John Casey), 'The Gallic Chronicle Restored: A Chronology for the Anglo-Saxon Invasions and the End of Roman Britain', Britannia19, (1988), pp.367–398; R.W. Burgess, 'The Dark Ages Return to Fifth-Century Britain: The 'Restored' Gallic Chronicle Exploded', Britannia21, (1990), pp.185–195
^"Town and Country: The End of Roman Britain", World Archaeology12(1), (June 1980:77–92); Simon T. Loseby, "Power and towns in Late Roman Britain and early Anglo-Saxon England" in Gisela Ripoll and Josep M. Gurt, eds., Sedes regiae (ann. 400–800), (Barcelona, 2000: pp. 319–370 (on-line text互联网档案馆的存檔,存档日期25 January 2012.) makes a strong case for the discontinuity of urban life.
^Philip Barker's excavation in the Baths Basilica at Wroxeter, (1975) is noted by R. Reece, "Town and country: the end of Roman Britain", World Archaeology, 1980.
^H.R. Loyn, Anglo-Saxon England and the Norman Conquest, 2nd ed. 1991:15f: "it is altogether unlikely that organized town-life can have survived through the troubles of the fifth and sixth centuries. Gildas lamented the destruction of the twenty-eight cities of Britain, and there is no reason to doubt the essential truth of his statement" (p16).
^A.S. Esmonde Cleary, "The Roman to medieval transition" in Britons and Romans: advancing an archaeological agenda. ed. S. James & M. Millett, (York: Council for British Archaeology, 2001)
^John Davey, "The Environs of South Cadbury in the Late Antique and Early Medieval Periods" in Debating Late Antiquity in Britain AD 300–700. ed. Rob Collins & James Gerrard, (Oxford: British Archaeological Review, 2004)
^Archaeological Resource Assessment of the Isle of Wight: Early Medieval period. Compiled by Ruth Waller, Isle of Wight County Archaeology Service, August 2006 Oxford Archaeology (页面存档备份,存于互联网档案馆).
^A. S. Esmonde Cleary, The Ending of Roman Britain, (London: Batsford, 1989), pp.138–139
^Helena Hamerow(英语:Helena Hamerow), 'The earliest Anglo-Saxon kingdoms' in The New Cambridge Medieval History, I, c.500–c.700. ed. Paul Fouracre (Cambridge: Cambridge University Press, 2005), pg. 265.
^Bury, J. B., A History of the Later Roman Empire from Arcadius to Irene, Vol. I (1889)
^Birley, Anthony R., The Roman Government of Britain, Oxford University Press, 2005, ISBN0-19-925237-8
^Jones, Arnold Hugh Martin, John Robert Martindale, John Morris, The Prosopography of the Later Roman Empire, volume 2, Cambridge University Press, 1992, ISBN0-521-20159-4
^Birley, Anthony Richard The Roman Government of Britain OUP Oxford (29 September 2005); ISBN978-0-19-925237-4, pp. 461–463
^
Halsall, Guy Barbarian Migrations and the Roman West, pp. 376–568 Cambridge University Press; illustrated edition (20 December 2007); ISBN978-0-521-43491-1, pp. 217–218
^Discussion in Martin Millett, The Romanization of Britain, (Cambridge: Cambridge University Press, 1990) and in Philip Bartholomew 'Fifth-Century Facts' Britannia vol. 13, 1982, p. 260
^H.R. Loyn, Anglo-Saxon England and the Norman Conquest, 2nd ed. 1991, p. 3.