To understand Arthur one must know what the world was like during and before the sixth century. In 400 BC the Celts migrated to Britain. They were able to overcome the natives of Britain because of their knowledge of iron tools, which the natives of Britain lacked. The Celts quickly spread their beliefs throughout England and Ireland. The Celts' government consisted of a collection of tribes, each with a king and a queen. One of the most important classes in Celtic life was the druids. The druids were the priests, judges, and healers of the society. It was the Celts who built monuments like Stonehenge.
In the first century A.D. Rome began its occupation of Britain. The Celts were no match for the drilled and well-equipped Roman army. Britain was now the Roman province of Britannia. The Celts lost the ability to defend themselves, and began to rely heavily on Rome's aid. Romans began to settle in Britannia, and introduced the Roman way of life. Christianity was also introduced. The Roman and Celtic society blurred together to create Britons.
Not everyone, however, was pleased with the Romans. Hadrian's Wall was built in Northern England to keep out the Picts and Celts from Scotland. The Visgoths, Germanic tribes driven out of their homeland by Asiatic tribes, began to invade the Roman Empire. Roman influence was weakening. In 410 A.D. the Visgoths, led by Alaric, captured the capital of the Roman Empire for a short while. Roman soldiers stationed in Britain were called back to defend Rome. By 500 A.D. the Roman Empire would collapse. The Britons were on their own.
Celts from Ireland, Picts from Scotland, and Saxons, Angles, and Jutes from Europe attacked Britain after Rome's retreat. The most dangerous of these groups was the Saxons. By A.D. 450 the Saxons had settlements in Southern England. The strongest Briton at this time was Vortigern, a chieftain from Wales. Vortigern agreed to pay the Jute leaders tribute if they would help defend Britain from the Picts and Scots.