The Magna Carta is an important document as it safeguarded the citizens rights. It was signed by King John in 1215. The charter established rule of law and limited king's power and made him accountable.He also agreed that no man would be punished or jailed without a legal procedure. He agreed not to impose any taxes without the consent of the Parliament. It mostly benefited the rich barons. It was the first time that a kings power was checked. Although the king remained ruler of England but absolutism was revoked.
Its a significant American event because of the rarity of a president being impeached. There have only been 2 impeached in History(3 if you count Richard Nixon but he resigned during Impeachment Trials)