Arthur's paternal uncle King Richard I had no children and before setting off on his most favourite pastime - the Crusades he named the boy heir to his English throne. However, in 1199 when Arthur was twelve Richard had a change of heart and made his last remaining brother John heir. Quite a sensible thing to do as a country with a minor on the throne is nothing but trouble.
Richard was dead by the end of March 1199 and despite his estate being in order, there was trouble anyway.
The barons of England supported Arthur - of course, they did !
England's hero the great William Marshall and John's mother Eleanor of Aquitaine stood on John's side.
It is interesting don't you think that the story of Arthur shows similarities to the disappearance of Princes in the Tower two-hundred years later?
Heir’s to the throne go missing, their wicked uncle does the dirty deed and then the bodies are lost forever.