The Magna Carta was written due to tension between the unpopular King and a group of rebel barons. It was written in Windsor, England and "it promised the protection of church rights, protection for barons from illegal imprisonment, access to swift justice, and limitations on feudal payments to the Crown, to be implemented through a council of 25 barons."
They were able to unite because the Persian invasion of Greek mainland was a common threat for all, so they united to stop it. This was impossible before because Persia was not considered to be such a major threat.
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