Born a second-tier member of the royal family on 21 April 1926, Elizabeth Alexandra Mary Windsor became the heir presumptive to the British throne following the abdication of King Edward VIII in 1936, followed by the coronation of her father, King George VI, in 1937. She only graduated from heir presumptive to heir apparent after it became clear that George would never produce a son. Many years later, she would be instrumental in changing the rights of succession to ensure that the first-born child of any monarch stood next in line to the throne regardless of gender. The legislation, rolled out across the Commonwealth, came into effect in 2015, one of many ways in which she modernised the Crown as an institution during her reign as the longest-serving monarch in British history.

Elizabeth’s beloved father, whose pronounced stutter and diffident nature almost resulted in his being passed over as monarch, trained her for her role from his coronation onwards – engaging Henry Marten, a provost at Eton College, to tutor her about the intricacies of British history and law from the age of 13. Based at nearby Windsor Castle, the young princess would visit Marten twice a week at his study near the college, where he would test her on the likes of The Law and Custom of the Constitution by William Anson in between tending to his pet raven – which he let fly around among his innumerable piles of books. Virtually all of the 14 Prime Ministers that the Queen subsequently worked with would remark on her in-depth knowledge of constitutional matters.

At home with her family, George would have the young Elizabeth sit with him while going through his famous red boxes of state correspondence – boxes that would one day become her responsibility. “I have a feeling that in the end probably that training is the answer to a great many things,” she said ahead of her 40th year on the throne. “You can do a lot if you are properly trained, and I hope I have been.” (Another legacy of her father’s? The Queen’s passion for corgis. He gifted her with her first Pembroke Welsh Corgi, Susan, for her 18th birthday, with Her Majesty keeping 14 generations of Susan’s descendants as pets.) Meanwhile, the Queen Mother, then Queen Elizabeth, encouraged her daughter to read literary classics by everyone from Jane Austen to Anthony Trollope, as well as to keep a nightly diary, just as her great-great-grandmother Queen Victoria did – a practice Elizabeth kept up until her death.

It’s also during this period that the future monarch established two of her greatest passions: riding and horse breeding. In a tradition dating back to Elizabeth I, the Queen bred her own horses, with operations at Sandringham among several other of her residences. Her lessons on horseback began at the age of three, and by the time she had turned 12 she had mastered side-saddle in preparation for the annual Trooping of the Colour ceremony, for which she would be required to lead 1,400 soldiers down the Horse Guards Parade in London. Many years later, while taking the parade down the Mall in 1981, a gunman fired a number of shots that turned out to be blanks at Her Majesty, startling her 19-year-old mare Burmese. As guards tackled the would-be assassin, the Queen quietly reached down, patted the skittish horse and finished the ceremony without comment.

Her experiences of the Second World War, which began a few months after her 13th birthday and lasted through most of her teenage years, further engrained her commitment to national service. The Princesses stayed in Windsor, sleeping in a makeshift bomb shelter underneath the 900-year-old castle whose foundations were laid by William the Conqueror, which remained without heating throughout the conflict, each room lit by a single bulb. Their mother promptly shut down the notion that the girls should be dispatched to Canada for their safety. “The princesses would never leave without me, and I wouldn’t leave without the King, and the King will never leave,” she declared flatly, then set about doing target practice in Buckingham Palace with a revolver gifted to her by Winston Churchill.

If her father gifted the Queen with her dedication, the Queen Mother is largely to thank for her self-possession. When the intruder Michael Fagan broke into Buckingham Palace in 1982 and made his way into the Queen’s bedroom, she calmly spoke with him about his troubles until the police arrived; 40 years earlier, a person suffering from mental illness had stolen into the Queen Mother’s room at Windsor Castle, hidden behind the curtains in her bedroom and grabbed her ankles. Rather than screaming, she looked down at him and said flatly, “Tell me about it,” before raising the alarm. In a similar vein, after half a dozen German bombs hit Buckingham Palace in 1940, the Queen Mother’s response spoke volumes. “I am almost glad we have been bombed. Now I feel I can look the East End in the face.”

Back in the relative safety of Windsor, Princess Elizabeth gave her first radio speech in 1940 on the BBC’s Children’s Hour programme – offering words of comfort to London’s many child refugees, who were dispatched to live with strangers in rural areas as the threat of the Blitz loomed. “My sister Margaret Rose and I feel so much for you, as we know from experience what it means to be away from those we love most of all.” The future queen also served in the Guides, practicing her wilderness skills in and around Great Windsor Park with East End refugees, and joined the Auxiliary Territorial Service, where she negotiated a three-tonne vehicle through heavy London traffic and learned how to change tyres and spark plugs. She would remain a skilled motorist for the rest of her life, comfortably driving off-piste on her Sandringham and Balmoral estates. In order to boost morale at Windsor, both Elizabeth and Margaret would star in pantomimes each Christmas in the Waterloo Chamber, performing for audiences of 500 servicemen or more.

On Victory in Europe Day on 8 May 1945, Elizabeth and Margaret were so overjoyed that the Princesses snuck out into the crowds gathered around Buckingham Palace and stayed out most of the night celebrating – gleefully dancing the conga through the streets in a rare moment of freedom. (The dance step is yet another legacy of her father’s; the King had a habit of ending parties at the Palace by leading conga trains through the state rooms.) The sisters repeated the act the following night, and again on 15 August, when Japan admitted defeat. Elizabeth wrote in her diary of the former outing, “Out in crowd again. Embankment, Piccadilly, Pall Mall, walked simply miles. Saw parents on balcony at 12:30am – ate, partied, bed 3am!” – while she noted she “ran through Ritz” and “drank in Dorchester” on the latter occasion.

The war also served to cement the Queen’s attachment to Lieutenant Philip Mountbatten, whom she had met at the age of just 13 on a visit to the Royal Naval Academy in Dartmouth with the royal family. While serving in the navy, Philip wrote frequently to Elizabeth, visiting her at Windsor Castle during his leave. During one notable Christmas period, Queen Mary, Elizabeth’s grandmother, became aware of the young couple’s attachment after spotting Philip watching Elizabeth lovingly during a performance of Aladdin. In spite of disapproval from various camps about the match, Philip proposed to Elizabeth during a visit to Balmoral in the summer of 1946, which she accepted at once without even consulting the King. The pair would formally share the news of their impending nuptials on 9 July 1947, shortly after Elizabeth’s 21st birthday, which she celebrated in South Africa with her mother, father and sister.

For the latter occasion – on her first international tour – Elizabeth gave a speech broadcast by the BBC around the world. “If we all go forward together with an unwavering faith, a high courage and a quiet heart, we shall be able to make of this ancient Commonwealth… an even grander thing – more free, more prosperous, more happy and a more powerful influence for good in the world,” she told hundreds of millions of listeners. “It is very simple. I declare before you all that my whole life, whether it be long or short, shall be devoted to your service.” During her first tour after her coronation, Elizabeth used her Christmas speech to reaffirm that the Commonwealth “bears no resemblance to the Empires of the past”, but is instead built on a foundation of “friendship, loyalty and the desire for freedom and peace” through an “equal partnership of nations and races”.