When tribes of Anglo-Saxon invaders began to settle in Britain after 450 A.D., the Britons who already lived there were pushed back into Wales, Scotland and Cornwall - even over the sea to Ireland and Brittany.
The real King Arthur may have been a leader of the Britons who fought against the Anglo-Saxons and managed to stop them for a while.
The Britons were a Celtic people, like many of the Irish, Scots and Welsh today. They had their own stories of gods, heroes and magic. King Arthur became part of these stories.
His name began to appear in books about the history of Britain. Later on, the Celtic storytellers of Brittany became famous, travelling through all of France with their tales of adventure and magic about Arthur and his group of knights. Soon these stories began to be written in French, and then in German, English and other languages. Before long all of Europe was talking about them.