The Archbishop of Canterbury Justin Welby, left, Prime Minister David Cameron, Britain's Queen Elizabeth and Prince Philip attend an event marking the 800th anniversary of Magna Carta in Runymede on June 15, 2015. (Stefan Wermuth/Reuters)

Britain marked the 800th anniversary of the Magna Carta, or "great charter," on Monday with pomp and solemnity. The British royals attended the ceremony, which celebrated the medieval document many consider to be the progenitor of modern republican principles. British Prime Minister David Cameron hailed the charter, a pact forged in 1215 between England's much-loathed King John and a clutch of rebellious barons, as forever shifting "the balance of power between the governed and the government."

The Magna Carta, which was originally known as the Articles of the Barons, has multiple versions, and its legacy has morphed over time. It's seen both as a foundational text of the English nation, and as a template for contemporary conventions governing international rights.

Cameron said the Magna Carta is still celebrated now because people "see how the great charter shaped the world, for the best part of a millennium, helping to promote arguments for justice and for freedom."

Why the Magna Carta is so significant:

1. The most important clauses in the Magna Carta are the ones that follow, excerpted from the British Library's English translation of the charter, which was written in Latin by England's largely French-speaking nobles.

+ (39) No free man shall be seized or imprisoned, or stripped of his rights or possessions, or outlawed or exiled, or deprived of his standing in any way, nor will we proceed with force against him, or send others to do so, except by the lawful judgment of his equals or by the law of the land.

+ (40) To no one will we sell, to no one deny or delay right or justice.

The barons wanted to curtail the ruling monarch's ability to act tyrannically against their interests -- from levying cruel taxes to seizing the property of dissenting nobles. In so doing, they made one of the Western world's first articulation of due process and the right to a fair trial.

2. The charter was more a peace settlement than a medieval proto-Bill of Rights. King John was compelled to find agreement with the rebellious barons, who had earlier laid siege to London, because his reign, and almost certainly his life, was in danger. Here was a reigning monarch, backed by supposedly divine mandate, being forced to negotiate and settle on actual terms with his own putative subjects. It was an unprecedented moment, at least in British history.

3. The story of the Magna Carta has particularly strong resonance across the pond. As Harvard historian Jill Lepore recounts in a recent article in the New Yorker, "the myth of Magna Carta as a single, stable, unchanged document contributed to the veneration of the Constitution" in the United States.

Numerous American states have incorporated either the full text of the document or parts of it in their statute books. The image of King John affixing his seal to the document is depicted in a mural in the U.S. Supreme Court, and the document has been invoked in the arguments of Supreme Court justices up to the present day.

A 1941 memorandum by the British Foreign Office mused over the special resonance the Magna Carta had in the U.S. "America was created in 1776 by a document; the most precious national relic they possess. That document is an affirmation of personal and national liberty, and it is, in American thinking, an 18th Century child of Magna Carta," it read.

What shouldn't be celebrated:

1. Not long after King John agreed to the Magna Carta, he sent it to the Pope in Rome with the hope that it would be nullified. The Vatican, which was probably as appalled at this subversion of sovereign authority as the weak British monarch was, duly obliged, annulling it by papal bull in August of 1215. The great contract between ruler and subjects, at least in its original form, lasted a matter of weeks before hostilities resumed once more. King John would die of dysentery the following year.

Moreover, the charter's principles were hardly novel. As Lepore notes, the "Magna Carta borrows from many earlier agreements; most of its ideas, including many of its particular provisions, are centuries old." Well before 1215, monarchs had been bound to just governance by ceremonial oaths, and the terms of their rule spelled out in writing. Myriad ancient cultures had their own particular traditions of law-giving and liberty.

2. Sure, it was written eight centuries ago, but it's hard to read the Magna Carta as some sort of early blueprint of modern democracy -- as many now consider it. Nowhere in its original text do words relating to democracy or a parliament appear. The charter refers to the rights of "free men," but the vast mass of the realm's inhabitants at the time did not belong to that category. Instead, far from the rights of all men, the Magna Carta reflected the interests of a tiny coterie of powerful men, who were mostly interested in consolidating and protecting their power.

3. Another feature of its time: The Magna Carta is conspicuously anti-women and anti-Semitic. One clause prohibits a woman from accusing a man of murder or manslaughter, while two others sought to place limits on the money owed specifically to Jewish moneylenders.

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