So at around 400 ce southern Britain was a peripheral part of the western Roman Empire, populated by the Britons, some of whom were Roman citizens. They were occasionally raided by the Gaels from modern Ireland, the Picts from Scotland, north of Hadrian's wall.

As the western Roman Empire began to collapse, its military forces were withdrawn, and by its full collapse in roughly 409, there was an era of mass migrations across europe.

From around 410, The Saxons, Angles and Jutes, germanic tribes occupying roughly modern Germany and Denmark, began a large scale migration into Britain and began to conquer or displace the local populations. The Angles occupied much of the east coast and central territories which eventually came to be known as the kingdoms of Northumbria, Mercia and East Anglia. The Saxons took over the south, forming the kingdoms of Essex, Middlesex, Sussex and Wessex. The Jutes really only occupied Kent and thus they are usually forgotten about and people just talk of the Anglo-Saxons. Only Wales remained as a Briton controlled territory.

Fast forward about 400 years to around 793 and suddenly a bunch of raiders from Norway, Sweden and Denmark appear on the east coast of Britain and start looting and pillaging. These are the Vikings. After only a few raids the Vikings left Britain alone and focus on Scotland and Ireland. In about 840 they establish a major settlement in Ireland called Dublin. In around 865 separate Viking bands from Denmark decide to unify and return to Britain not to raid but to conquer. So, after being left alone for about 40 years Britain is suddenly invaded by the 'Great Heathen Army' which over the next two decades occupies eastern and central Britain creating a territory known as the Danelaw.

In around 900 Æthelstan of Mercia first unified all the anglo-saxon kingdoms and by 937 he went on to defeat the Scots, Danes and Vikings, creating a unified Kingdom of England.

While all this was going on other Vikings had been continuously raiding all across the northern european coast, into Russia and the black sea and even making it as far as Sicily. France began to be raided around 800ad and between 840 and 860 Paris itself was attacked multiple times. Unable to halt the raids in 911 Charles III made a treaty with a viking king Rollo which essentially was about using vikings to stop the other vikings. Charles gave Rollo control over the territory which became known as Normandy in return for a vow to defend the estuaries of the Seine from further viking attack, and oaths of vassalage and baptism.

In 1015 Cnut, the King of Denmark again invaded England and by 1017 he was king of England, Denmark and Norway.

After Cnuts death there were a series of struggles over succession of the English throne and eventually in 1066 William of Normandy, who had some claim to the throne, invaded England and defeated the other claimants, was crowned King of England and became known as William the Conqueror.

The Norman Kings of England slowly start to see themselves as English... and we eventually get to where we are now.