Prince Rainier of Monaco was in danger of losing control of his principality after World War II, so advisors dreamed up a plan that eventually led him to wife Grace Kelly

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The glamorous Monégasque royals' position atop their throne hasn't always been guaranteed.

In the new Curiosity Stream documentary Royals: Keeping the Crown, Prince Rainier of Monaco hits a crisis point in his reign that almost gave the world Princess Marilyn instead of Princess Grace.

Shortly after World War II, Monaco — like so many other European countries — was financially and culturally ravaged, and a longstanding agreement with France meant the prince's crown was in danger of being lost if Rainier couldn't revive his country's primary industry of tourism. Add to that, Rainier was also an unmarried man who had yet to produce a son to inherit his seat on the famous gambling destination's throne.

According to University of St. Andrews historian Dr. Chandrika Kaul, "Prince Rainier is now 32, and the idea that he might lose his principality if he doesn't marry well and produce a male heir is uppermost in his mind."

Adds Royal historian Kate Williams, "The stakes couldn't be higher — Monaco is under threat. It could collapse, and Rainier has to save it."

Unexpectedly, the prince's advisors suggested an out-of-the-box idea: Rainier should merge Monégasque nobility with America's "new royalty" by wooing a silver screen starlet.

"Prince Rainier was advised that he would do well to marry a Hollywood 'princess,' and [capitalize on] all the celebrity and fame that went with that marriage," says Dr. Kaul. "What Monaco needed was to revive its tourism trade. It made perfect economic sense."

Enter Marilyn Monroe — almost...

Prince Rainier, mailyn monroe
Credit: getty (2)

According to the Keeping the Crown, Monroe was the first suggestion on the prince's list — but the Some Like It Hot star had an overly sensual image, which struck her from the list.

Eventually Rainier found a Country Girl in actress Grace Kelly, when the two-time Oscar winner was in the region for the 1955 Cannes Film Festival. She was invited to Monaco by Paris Match for a photo shoot at the royal palace that just so happened to be hosted by a regal tour guide in Prince Rainier himself.

"The executive at Paris Match think that this will be a really dramatic, sensational meeting between the Queen of Hollywood and their own Prince, who is an eligible, young, handsome bachelor — who also just happens, rumor has it, to be looking for a wife," says Dr. Kaul.

Adds Williams, "During this photo shoot, you really see how Rainier is entranced by Grace, and really, I think, he falls in love with her on a personal level — but he also sees her as the answer to so many of his problems."

Prince Rainier
Credit: Keystone/Getty

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Royal watchers know how this story played out, with the prince sweeping the starlet off her feet and the couple marrying less than a year later on April 18, 1956.

Prince Rainier and the newly minted Princess Grace went on to have three children: Princess Caroline, Prince Albert and Princess Stephanie.

"It was such an incredible affair, and it's left such a mark on people," Albert told PEOPLE in 2017 of his parents' wedding. "What it has meant for people has been incredible. For us, it was ... our parents getting married. But what it's meant for Monaco, for people around the world, and, how their story continues to fascinate people, that's something unimaginable."

All six episodes of Royals: Keeping the Crown debut on Curiosity Stream Thursday at 9 a.m. ET.