Richard, the Lionheart
Richard, the Lionheart, King of England had spent much of his reign outside England fighting wars in the Middle East and France. To pay for these he had taxed the English heavily. In 1199, Richard died and his brother, John became king.
John continued to fight wars in France but he kept losing battles. He needed more money so his government in England ruthlessly demanded more taxes from the nobility who were expected to pay tax if the King asked.
The Barons became very unhappy about John exploiting their loyalty and belief in his complete power. They rebelled and took over London and forced John to negotiate.