As Britain’s longest-reigning monarch, Queen Elizabeth II has devoted her life to the service of her country. But nearly a decade before she became queen, she entered another kind of service when she became the first female member of the British royal family to become an active duty member of the British Armed Forces — making her the last surviving head of state to have served during World War II.

Elizabeth led a very sheltered life up until WWII

When Elizabeth was born in April 1926, she wasn’t destined for the throne. Her father Albert, known as “Bertie” to his family, was the second son of King George V. Elizabeth and her younger sister Margaret were doted on by their parents, with her father dubbing the family, “we four.” It was a comfortable but strictly confined upbringing.

Elizabeth’s world was turned upside down in 1936, following George V’s death and the abdication of Elizabeth’s uncle Edward VIII just months later. Edward’s decision to cast aside the crown to marry Wallis Simpson left her father, now King George VI, on the throne and 10-year-old Elizabeth as heir presumptive.

READ MORE: Why Edward VIII Abdicated the Throne to Marry Wallis Simpson

World War II was a pivotal moment for the royal family

George VI struggled to find his footing after assuming the throne. Tentative, nervous and prone to both ill health and a relentless stutter, he found public speeches and appearances agonizing. But, as the war began, he found his footing. Bolstered by his wife and government officials, including Prime Minister Winston Churchill, the king helped rally the morale of the British people, refusing to leave London despite repeated aerial attacks by Nazi planes that left cities in ruins. He and the queen regularly visited those most devastated by the attacks, and when one bombing in September 1940 badly damaged a portion of Buckingham Castle, bringing the dangers of the war to the royal’s doorstep, the queen noted, “Now I can look the East End in the face.”

With the looming threat of a German invasion, the British government urged the queen to leave for Canada with her daughters. She refused, stating, “The children won't go without me. I won't leave without the King. And the King will never leave." But, like thousands of other children who were evacuated from British cities, Elizabeth and Margaret did leave London, spending much of the war at Windsor Castle. In 1940, the two sisters made one of their first public addresses, recording a radio broadcast to their fellow children in Britain and British colonies and dominions around the world. 

Elizabeth had to fight to join the military

With the British people rallying to cause, Elizabeth, like many other young Britons, yearned to do her part. But her protective parents refused to allow her to enlist, noting that no female member of the royal family had ever joined the military. Elizabeth was a dutiful daughter, but she was also strong-willed. It took more than a year of debate before her family finally relented in early 1945, giving the now 19-year-old permission to join up.

That February she joined the Women’s Auxiliary Territory Service (similar to the American Women’s Army Corps or WACs), registered as inductee No. 230873, under the name Elizabeth Windsor. The ATS provided key support during the war, with its members serving as anti-aircraft gunners, radio operators, mechanics and drivers.

READ MORE: The Dramatic Differences Between Queen Elizabeth and Princess Margaret

The sheltered princess took to her new role with gusto

She underwent a six-week auto mechanic training course at Aldershot in Surrey, and by July had risen from the rank of Second Subaltern to Junior Commander. She learned how to deconstruct, repair and rebuild engines and change tires, and learned how to drive every type of machine she worked on, including jeeps, trucks and ambulances. As a 1947 Collier’s magazine article noted of the overalls-clad teen, “One of her major joys was to get dirt under her nails and grease stains in her hands, and display these signs of labor to her friends.”

For the first time, Elizabeth worked alongside her fellow Brits, relishing the freedom she had previously been denied. But there were concessions to her rank. She ate many of her meals in an officer’s mess hall, not with other enlistees. And each night she was driven home to the safety of Windsor Castle.

The Associated Press dubbed her 'Princess Auto Mechanic'

Elizabeth took her ATS duties seriously, but the idea of a future queen repairing automobiles proved irresistible. Her enlistment made headlines around the world, with newspapers applauding the young princess’ commitment to the war effort and calling her "Princess Auto Mechanic." And though her parents had proved resistant to the idea of her joining the military, they were immensely proud of their daughter, paying a visit to her ATS unit in April 1945, with younger daughter Margaret — and a fleet of reporters and newsreel cameramen — in tow.

Elizabeth was still in the ATS when Germany surrendered on May 8, 1945, and she and Margaret famously slipped out of Buckingham Palace to join revelers celebrating in London. Her military service ended with Japan’s surrender later that year. The war was over, but it had taken its toll on the royal family. George VI had helped lead his country in its darkest hour, but his health (thanks in part to a lifelong smoking habit) would never be the same.

Queen Elizabeth II

Queen Elizabeth in 2007

The queen remains a fan of cars today

In 1947, the usually deferential Elizabeth once again fought to get her own way. This time, she was determined to overcome the objections of her family and the British government to marry Philip Mountbatten, a Greek-born, dashing young officer in the Royal Navy, who Elizabeth had fallen for at the age of 13. That November, with Britain still in a post-war economic downturn resulting in rationing, the couple married. Several of their children and grandchildren would follow in their footsteps by serving in the armed services, including Prince Charles, Prince Andrew, Prince William and Prince Harry, whose 10-year stint in the British Army included two tours in Afghanistan.

And while the Queen is best known as a keen and passionate rider and breeder of horses, her teenage years in the ATS left their mark. She taught her children how to drive, and well into her 90s can frequently be seen behind the wheel of one of the many cars in the royal collection — and she’s been known to jump at the chance to diagnose and repair faulty engines and machinery, just as she was trained to do more than 70 years ago.