Fly out of London's Heathrow Airport and you may pass over a grassy field called Runnymede. Eight hundred years ago this month, it offered a colorful spectacle, dotted with the tents of barons and knights, and the larger pavilion of King John of England, looking like a circus top with the royal standard fluttering above.

Despite the gathering's pageant-like appearance, the atmosphere was undoubtedly tense. The purpose was to settle a conflict between rebellious barons and their king, a ruler described by a contemporary as "brimful of evil qualities."

John's efforts to raise money to regain lost lands in France exceeded the usual taxes and levies that the nobles had accepted from his predecessors. The king seized the estates, and sometimes the person, of wealthy lords or merchants and demanded hefty payments for their release.