Nelson Mandela said he was inspired by their love story, calling the nation they led "a shining beacon of light and inspiration to the rest of us in southern Africa."

Mandela was speaking of Seretse Khama and his wife, Ruth Williams Khama, who helped Botswana make a smooth, peaceful transition in the mid-1960s from a tiny British protectorate into a thriving democratic state -- and who went on to serve as the country's first president and first lady.

But the interracial couple's courtship and marriage was far from smooth. The son of a powerful tribal chief in what was then called the Bechuanaland Protectorate, Seretse scandalized both his people and the British government when he married Ruth, a white, middle-class office girl he met at a dance in London, where he was studying law.

Their 1948 wedding sparked an international diplomatic crisis.

Their story is told in A United Kingdom, an adaptation of Susan Williams' biography Colour Bar: The Triumph of Seretse Khama and His Nation  starring David Oyelowo (Selma, Queen of Katwe) as Seretse and Rosamund Pike (Gone Girl) as Ruth.

Already a critical hit in England, where it has been hailed as the British counterpart to the Oscar-nominated civil rights romance Loving, A United Kingdom opens Feb. 17 at the Ritz East.

Oyelowo and Pike talked about the film in separate phone interviews.

Oyelowo, 40, who was also a producer on the film, said he considers Seretse and Ruth's romance one of the greatest love stories of the 20th century. Yet, like most Brits, he'd never heard about it.

"I just couldn't believe I didn't know their story," he said.

"And as a person of African descent, who has himself lived there – I lived in Nigeria for seven years – I said to myself this is exactly the kind of narrative I wanted to see cinematically."

Oyelowo said Seretse and Ruth's happy marriage has a deeper, more personal resonance he couldn't ignore: His wife of 19 years, actor Jessica Oyelowo (Alice in Wonderland), is Caucasian.

She's featured in A United Kingdom as the rabidly racist wife of a British functionary played by Jack Davenport.

"I think she has a little too much fun with that role," David Oyelowo said, laughing. "We met as theater students when we were 17, and while we had a few run-ins [with racist thugs] on the street, we've never had entire nations go up against our marriage."

Oyelowo said casting Ruth's role was a no-brainer. He immediately reached out to Pike, with whom he shared the screen in the Tom Cruise vehicle Jack Reacher.

"Our one interaction was when [my character] used a Taser on her in an elevator," he said. "But I've been a fan of Rosamund for a long time and I felt she really has … that enigmatic quality that I think Ruth needs."

Pike, 38, said Seretse and Ruth's romance affected her on a visceral level.

"I opened a book of photographs [Oyelowo] sent me and I just looked at the faces of this man and this woman and I felt an immediate emotional connection to them," she said. "I'd never had a reaction like that before. I thought, well, there's something very profound being transmitted to me from these photos and I knew I had to play this."

Added Pike, "I mean, I had started crying by this point."

A United Kingdom charts the ugly political repercussions of the couple's marriage. Members of Seretse's tribe and the British government conspired to break it up. When Seretse refused to divorce Ruth, he was exiled from his African homeland for five years. In England, politicians made hay of the sanctions and Conservative Party leader Winston Churchill ran against the Labor Party with a promise he'd return Seretse home. Yet once his party was in power, Churchill made the exile permanent, banishing Seretse for the rest of his life. (The couple did finally return to Bechuanaland in 1956.)

Ruth, then pregnant with their first child,  decided to stay in the sparsely populated African territory. She wanted the child, a girl, born in her rightful home. (Ruth and Seretse would have three more children.)

Oyelowo said he lobbied for director Amma Asante (Belle, A Way of Life) to helm the picture because she immediately grasped how best to balance the story's political and romantic aspects.

"Amma told me straight off anything political that happened in the film had to be something that drives the love story forward and not the other way around," said Oyelowo.

He said he's proud the film was shot entirely on location in Botswana.

"The house we used was the actual house where they lived," he said. "And Ruth's labor scene was shot in the hospital where Seretse was actually born. … The authenticity lends the film an atmosphere that's undeniable."

While Oyelowo is anxious to find out how American audiences will react to the film, he said he was gratified that Seretse and Ruth's son, Ian Khama, who happens to be Botswana's current president, loved the film.

"He made an unannounced visit to the set by helicopter," said Oyelowo. "He sat down behind me and we watched Rosamund on-set playing his mum."

"He turned to me and said, 'I never thought I would see my parents again.' "