what is the purpose of this constitutional document?
To preserve liberties and rights
Magna Carta
 to preserve liberties and rights that they believed to be enshrined in Magna Carta.
how did the magna carta advance the cause of liberty Magna Carta still forms an important symbol of liberty today, often cited by politicians and campaigners, and is held in great respect by the British and American legal communities, Lord Denning describing it as "the greatest constitutional document of all times – the foundation of the freedom of the individual against the arbitrary authority of the despot". what is it? A Constitutional document it was first drafted by whom? King John
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Magna Carta [SEP] Use in the Thirteen Colonies and the United States
The development of parliamentary supremacy in the British Isles did not constitutionally affect the Thirteen Colonies, which retained an adherence to English common law, but it directly affected the relationship between Britain and the colonies. When American colonists fought against Britain, they were fighting not so much for new freedom, but to preserve liberties and rights that they believed to be enshrined in Magna Carta.