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LONDON — Princess Elizabeth was never meant to become queen, let alone the longest-serving monarch in the storied history of British royalty.

Her father, in fact, was never meant to become king.

But on Monday, after nearly a lifetime of service to country and crown, Elizabeth II will commemorate her 65th anniversary as queen. She will become the only British monarch ever to celebrate her Sapphire Jubilee.

It is a role that most Britons — whether royalist or republican — would agree that she has fulfilled with caution, dignity and an unending sense of duty.

Then-Princess Elizabeth and Philip, Duke of Edinburgh, pose with King George VI and other members of the royal family after their wedding on Nov. 20, 1947. Topical Press Agency/Hulton Archive / Getty Images - file

In keeping with the habits of a lifetime, Elizabeth, who is 90, is expected to spend the day quietly at Sandringham, her country estate 110 miles north of London.

The course of Elizabeth's life was forever changed in 1936 when King Edward VIII, her uncle, abdicated to marry Wallis Warfield Simpson, a twice-divorced U.S. commoner. His brother Albert became king, adopting for the name George VI.

Queen Elizabeth II returns to Buckingham Palace after addressing Parliament on Nov. 18, 2009. Leon Neal / AFP - Getty Images - file

And Princess Elizabeth, who had spent the first 10 years of her life never expecting to become queen, suddenly found herself first in line to the throne.

With Edward's abdication, it fell to George VI — a shy man and a stutterer — to inspire his countrymen to endure the hardships of World War II and to prevail. He was a smoker, and the strain took a toll on his health.

In February 1952, with her father ill, she and Prince Philip, her husband of five years, stood in for the king on an official visit to Kenya.

The couple took a brief and exhilarating rest at Treetops Hotel, nearly 6,500 feet above sea level, with a view of Mount Kenya.

"She has always kept her cards held close to her chest."

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On the night of Feb. 5-6, the king died. Although she did not know it, Elizabeth was already queen — the first British ruler of modern times never to know the exact time of her assumption of office, because the king died in his sleep.

"For the first time in the history of the world," her bodyguard, a hunter named Jim Corbett, wrote in the visitor's log book, "a young girl climbed into a tree one day a princess and after having what she described as her most thrilling experience she climbed down from the tree the next day a queen — God bless her."

Only, she did not know that until later in the day. At a hunting lodge, Philip took the call. Her father had died, he told his wife, and she was already queen. She was 25 years old.

Upon returning to England, she met on Feb. 8 with the Lords of the Council for the formal proclamation of her reign as Her Majesty Queen Elizabeth II.

"By the sudden death of my father I am called to assume the duties and responsibilities of sovereignty," she said. "My heart is too full to say more to you today than I shall always work, as my father did throughout his reign, to advance the happiness and prosperity of my peoples."

In her early days as monarch, the queen relied on her husband and others to help her feel her way.

"The queen had a range of advisers right from the get-go," said Roya Nikkhah, the royal correspondent for The Sunday Times. "She had Winston Churchill as her first prime minister, an amazing relationship between the old prime minister at the end of his tenure and the new monarch at the beginning of hers."

But the young queen had views of her own, as well. Despite advice to the contrary, she decided that that her coronation, on June 2, 1953, would be televised.

It was an important move, Nikkhah said.

The 1953 coronation of Queen Elizabeth II was the world's first major international event to be broadcast on television. Ronald Startup / Picture Post/Getty Images - file

"Televising the coronation was groundbreaking for its time — to bring the monarchy into millions of peoples' homes against all of the advice of her advisers, who said this makes the monarch look too day-to-day, too real," Nikkhah said. "She realized, actually, this is what she wanted to do, set the tone for her entire reign, making the monarchy relevant and bringing it to the people."

Further televised royal spectacles followed, such as the lavish 1981 wedding of Prince Charles — who has now been heir to the throne longer than anyone in British history — and Lady Diana Spencer.

And television is giving new life to those early days of Elizabeth's reign. The captivating young queen is portrayed in the successful Netflix series "The Crown," which has introduced those exciting early years to a new generation.

David Bailey photo shows Britain's Queen Elizabeth II posing wearing a suite of sapphires given to her by King George VI as a wedding gift in 1947. DAVID BAILEY / AFP - Getty Images

"Throwing back to her as a very young queen — because now we are so familiar with her as a nonagenarian — I think that has probably drawn a lot of people to new interest in the monarchy, who are able to see what this woman has done, what she has achieved, from a very young woman through her 90s," Nikkhah said.

The queen, now perhaps the world's most prominent nonagenarian, had a very different persona 65 years ago, said Camilla Tominey, the royal editor for The Sunday Express.

"If you look at the queen now, you see an elderly lady, and we have done for decades now. [But] she was such a glamorous figure, along with her sister, Princess Margaret — these women who radiated beauty."

Decades of duty have defined the queen. She has traveled more than a million miles, visited about 120 countries and met with 12 U.S. presidents, from Dwight D. Eisenhower to Barack Obama. And at the age of 90, she continues to perform her royal duties, carrying out 80 public engagements in 2016.

President Barack Obama and Queen Elizabeth II during a state banquet in Buckingham Palace on May 24, 2011. WPA Pool / Getty Images

She has served with dignity. A major part of her job, not to put too fine a point on it, has been to go 65 years without publicly saying anything inappropriate. It is a task at which her husband, Prince Philip, who is 95, has occasionally failed, with his penchant for ethnic jokes that are awkward at best.

One wobble, though, was the queen's failure to react quickly to Diana's death in a car crash in 1997. Diana and Charles were divorced, the queen was not thought to hold Diana in high regard and the flag at Buckingham Palace was not flown at half-staff.

However, the queen recovered her footing and made a speech expressing admiration for Diana and her grandmotherly concern for Princes William and Harry.

After so many years on the throne — longer than most Britons have been alive — many people have come to respect her quiet devotion to duty and her determination to keep her private life out of sight.

Even after 65 years on the throne, during a televised reign in which everyone came to know her love of horses and corgis, people know little about the queen as a person.

"With the queen, you have this dichotomy — that there was this very public figure, the most photographed woman in the world, combined with somebody who behind palace gates we didn't know much about at all as a woman," Nikkhah said. "She has always kept her cards held close to her chest."

The country is celebrating this anniversary by issuing a new 5-pound coin. It will bear the words the queen uttered nearly 70 years ago, the day she turned 21: "My whole life, whether it be long or short, shall be devoted to your service."