Mary Bowerman

USA TODAY Network

Magna Carta, which turns 800 years old on Monday, established the principle that no man was above the law, not even the king. The charter is revered as a cornerstone of democracy, with famous principles like the right to justice and a fair trial stemming from its inception.

With only four of the original copies from 1215 left, the document is priceless, well almost. In 2007, a somewhat younger copy from 1297 sold for $21.3 million at auction.

But in 1941, as World War II raged in Europe, the Magna Carta was also a potential bargaining chip. Britain hoped that offering Magna Carta to the United States would encourage the reluctant country to join the war, or at least continue supporting the British war effort, according to Jessica Nelson, medieval records specialist at The National Archives in the United Kingdom.

"[The offer] shows that Magna Carta certainly isn't just about Britain but is an important part of the American history as well," Nelson said.

Though the proposal to gift the document to the U.S. eventually failed, here's an explainer on why the British wanted to give it to the U.S., and why it may have worked.

Magna Carta in America:

Magna Carta made its first public appearance outside of Europe at the 1939 World's Fair in New York, according to the Library of Congress. The document was displayed in the British pavilion, which was one of the largest at the fair. When the World Fair closed in October 1939, the British allowed the document to stay in the U.S., to avoid shipping it overseas during wartime, according to the Library of Congress.

By 1941, the document had been in America for two years and "many thousands of Americans…waited daily in long queues to view it with a reverence and pride scarcely believable to those who have not" seen it themselves, according to a British Foreign Office document with annotations by Winston Churchill.

Britain knew just how highly esteemed the Magna Carta was by Americans. In the Library of Congress, the document was exhibited opposite the U.S. Constitution and the Declaration of Independence, according to the library's website.

And the British Foreign Office noted that "in some respects Magna Carta has a more vivid appeal for the average American than for the average Englishman."

"America was created in 1776 by a document; the most previous 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," the Foreign Office minutes noted.

Was Magna Carta powerful enough to send America to war?

Before the attacks on Pearl Harbor, America was reluctant to join World War II, which was viewed as a foreign war. But the British desperately needed American aid to fight the German Nazi regime that was sweeping across Europe. With little hope that America would join the war effort, the British believed the Lincoln copy of Magna Carta would stand as a reminder "of the history and traditions which the two countries have in common," according to the Foreign Office document.

Britain thought "the gift of this document might be a powerful enough thing to persuade a country like America to go to war," Claire Breay, curator of the Magna Carta exhibition at the British Library told the AP.

"It's incredible," she said. "At the point at which the freedoms set out in Magna Carta are under threat, the government thinks about turning to Magna Carta."

Offering the Magna Carta to the United States would be viewed as the "most gracious of acts in American eyes," the Foreign Office wrote."It would represent the only really adequate gesture which it is in our power to make in return for the means to preserve our country."

Why it failed?

The idea was abandoned in part because the British government did not own Magna Carta they were planning to offer the United States, according to Nelson. The British Library offered to give one of its copies of Magna Carta to the Lincoln Cathedral in exchange for the one given to the U.S., but was denied.

Just months later, on December 7, 1941, America no longer needed a nudge to enter the war. The attacks on Pearl Harbor ushered the United States into the war, and the Magna Carta was returned to the Lincoln Cathedral in 1946.

Read theBritish Foreign Office document from March 16, 1941:

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