Following the Roman withdrawal in the 5th century, Britain fragmented into numerous small kingdoms, known primarily through folklore and archaeological findings. Wars erupted among the Britons, further exacerbated by raids from the Picts. The legend of Brutus of Troy showcases the Britons' desire to connect with Roman heritage, while the breakdown of Roman infrastructure left the island vulnerable.

Germanic invaders began arriving, with the Jutes (led by Hengist and Horsa) establishing the Kingdom of Kent by 459 AD. This was followed by the Saxons, who founded Sussex (Southern Saxons) in 477 AD under Ella, and Wessex (Western Saxons) in 519 AD. The Angles, migrating from Jutland, established Lindsey around 480 AD, and later Mercia, Bernicia, and East Anglia, and Deira by 560 AD.

The native Britons resisted the invaders, notably led by Ambrosius Aurelianus, who defeated the Saxons at the Battle of Badon Hill in 516 AD, temporarily halting their advance. The Kingdom of Dumnonia also fought constant wars to prevent westward Saxon expansion. The Anglo-Saxon invasions led to the creation of the King Arthur legend, a symbol of British resistance and heroism.

The Germanic peoples established societies based on tribal alliances and military democracy, with Kings tracing their lineage to pagan gods, and a social structure including "chorals" (free peasants) and "Earls" (nobility). Over time, the languages and traditions of the Saxons and Angles merged into a unified Anglo-Saxon culture. Christianity returned to the island in 597 AD when missionaries from the Pope persuaded King Ethelbert I of Kent, leading to Canterbury becoming a religious center and the gradual Christianization of the Anglo-Saxons. The term "Angland" or "England" first appeared around this time.

Ethelfrith of Bernicia became king in 593 AD, reforming the army and defeating the Welsh and Scots, expanding his kingdom. He later united Bernicia with Deira to form Northumbria in 604 AD. Edwin became king of Northumbria in 617 AD, annexing Welsh kingdoms and Lindsay, becoming the most powerful king in Britain. However, Edwin was defeated and killed in 633 AD by an alliance of Mercia and Welsh forces led by Penda and Cadwallon, leading to a temporary decline of Northumbrian power.

Oswald, son of Ethelfrith, reclaimed Northumbria after defeating Cadwallon at the Battle of Heavenfield. He further expanded the kingdom's borders. Oswald was later killed in 642 AD by King Penda of Mercia at the Battle of Maserfield. Oswald's brother, Oswiu, eventually reunited Northumbria and expanded its influence, installing a puppet king in Mercia after defeating and killing Penda at the Battle of Winwaed in 655 AD.

Penda's son, Wulfhere, abandoned his father's anti-Christian policies and, with the Church's help, rebelled against Oswiu, reclaiming Mercian independence in 658 AD. Wulfhere expanded Mercian influence, taking control of Middlesex and other territories. His brother, Ethelred I, continued Mercia's expansionist policies, engaging in conflicts with Kent, Lindsay, and Wessex, further shaping the political landscape of Anglo-Saxon Britain before the arrival of the Vikings.