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(英語: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(英語: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(英語: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(英語:Gildas)認為軍閥沃蒂根(英語:Vortigern)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(英語: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(英語: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(英語: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。
在6世紀晚期,撒克遜人再一次出現了擴張時期,從日後統治威塞克斯王國的王族於552年佔領威爾特郡的Searoburh (現今的老沙倫)(英語:Old Sarum)為起點,擴張包括在577年迪勒姆戰役(英語:Battle of Deorham)後進入中南至西南部的科茲窩地區,雖然對《盎格魯-撒克遜編年史》中這一時期的條目的準確性被受質疑。但現代作家經常在沒有明確證據的情況下,說這些征服將西南英格蘭的布立吞人(後來稱為西威爾斯人)與威爾斯的布立吞人分割開。(就在討論該時期之後,於611 年的車士打戰役(英語:Battle of Chester)可能已經將後者與英格蘭北部分割開。)但直到570年代,布立吞人仍然控制著英格蘭和威爾斯大約一半的地區。
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(英語:Michael Jones) andJohn Casey(英語: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互聯網檔案館的存檔,存檔日期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 (頁面存檔備份,存於互聯網檔案館).
Helena Hamerow(英語: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.
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
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