She was growing up. And then, on July 22, 1939, at the age of 13, she fell in love. The object of her admiration was a royal exile five years her senior, “a fair-haired boy, rather like a Viking, with a sharp face and piercing blue eyes.”

He was Prince Philip of Greece and Denmark and, like Elizabeth a great-great-grandchild of Queen Victoria. The Royal Family was on a visit to the Royal Naval College in Dartmouth, and the young naval cadet was put in charge of entertaining the Princesses while their father conducted an inspection.

After playing with them with a train set and eating ginger biscuits, the energetic Prince suggested they watch him on the tennis court. There, he showed off by jumping over the net, and Crawfie noted of Elizabeth: “She never took her eyes off him the whole time.”

The young Princess later marvelled to her governess, “How good he is, Crawfie. How high he can jump.” Elizabeth would later confirm in her father’s official biography that she had been in love with Philip “from their first meeting”.

After war was declared and Philip began his wartime naval service, he and Elizabeth would write to one another, and Philip spent his time on leave with the Royal Family. Elizabeth and Margaret had been at a service at Crathie church near Balmoral when Prime Minister Neville Chamberlain announced on September 3, 1939, that Britain was at war with Germany.

After Christmas, Elizabeth and Margaret moved to Windsor, which was deemed far safer than London. The Princesses’ contributions to the war effort were assiduously publicised, including giving presents of chocolate to evacuated French children and donations to the Over-Seas League Tobacco Fund to supply servicemen with cigarettes.

Even Elizabeth’s 14th birthday in April 1940 conformed to the new narrative of cheerful stoicism: Her birthday cake was a plain sponge, with no icing. Over the course of 1940, as the war escalated, the King and Queen were frequently at Windsor, spending nights at the castle, although the Royal Standard flew at Buckingham Palace to maintain the reassuring illusion of their presence in the capital.

Meanwhile Elizabeth and Margaret were “no longer…allowed to appear in public for reasons affecting their own safety”. Still, a suggestion that the Princesses should spend the war in Canada like members of the Dutch and Norwegian royal families was firmly rebuffed by the Queen.

Instead, an air raid shelter was built under Windsor Castle’s Brunswick Tower, with reinforced walls and a 4-foot-thick roof of concrete and girders. Then in October, with the Blitz under way, Elizabeth made her first radio broadcast, on the BBC Children’s Hour.

The speech, transmitted across the world, addressed the children sent away from their families to safety. Elizabeth spoke to them as one who knew “from experience what it means to be away from those you love most of all.” It was a triumph.

That Christmas, the sisters appeared in the castle’s nativity play, with Elizabeth playing the part of a king. In the audience, her father wept throughout. In February 1942, he gave his daughter an early 16th birthday present, appointing her Colonel of the Grenadier Guards.

Elizabeth inspected the regiment for the first time on her birthday. A newspaper reporter detected “no sign of nervousness, while Elizabeth herself described the experience as “a bit frightening…but not as bad as I expected it to be.”

Meanwhile, she was relishing every visit Philip made to the family on his wartime leave. By September 1942 she was writing to Crawfie that he was “The One”. Philip spent the Christmas of 1943 at Windsor. Crawfie thought 17-year-old Elizabeth “sparkled” throughout his visit like she had never done before.

But her parents still did not take it seriously. In March 1944, the King wrote to his mother that Elizabeth was “too young for that now”. The following month, the Princess turned 18, and clamoured to be allowed to carry out war work like many of her friends were doing.

But George VI regarded Elizabeth’s training for the crown as more important than any possible role in ordinary national service.

Soon, she was taking her first serious steps in public life, including making speeches at the Queen Elizabeth Hospital for Children and on behalf of the NSPCC. However, finally in March 1945 the King capitulated to his daughter’s entreaties.

And so she began an NCO cadre course for the Auxiliary Territorial Service at No.1 Mechanical Transport Training Centre in Camberley, Surrey, as Second Subaltern Elizabeth Alexandra Mary Windsor.

Looking back 50 years later, the Queen would say she had “learned a little about driving and the workings of the combustion engine and much about the strength and happiness of comradeship”. Meanwhile, when her aunt Princess Mary was scheduled to make an official visit to the training centre, the Princess wrote to Crawfie: “You’ve no idea what a business it has been.

“Everyone working so hard – spit and polish the whole day long. Now I realise what must happen when Papa and Mummie go anywhere. That’s something I shall never forget.”

But just weeks after Elizabeth’s 19th birthday, her ATS career came to an end when Britain celebrated victory in Europe. Elizabeth wore her uniform on the balcony of Buckingham Palace to greet the joyous crowds with her family.

And that night Elizabeth and Margaret left their parents to slip out with a small group and join the celebrations beyond the palace railings. Decades later, she remembered it as “one of the most memorable nights of my life”.

The sisters escaped again the following evening. Elizabeth wrote in her diary: “Trafalgar Square, Piccadilly, Pall Mall, walked simply miles. Saw parents on balcony at 12.30am – ate, partied, bed 3am!”

On the first night, she and her sister had gone out unrecognised, but that second night, 15-year-old reveller Ronald Thomas realised who Elizabeth was. He promised he would not tell anyone, and he danced with her in Trafalgar Square.

• The Sun report