What becomes of the 'spare's spare'?
It is no more the time of Joan of Arc, wrote Queen Marie of Romania in her diary on 27 August 1916, the day Romania entered the First World War on the side of Britain, France and Russia. Within three months, enemy troops had occupied Bucharest, Marie and her husband Ferdinand had been forced to flee to the north and, a year later, against the royal couple's wishes, Romanian politicians signed a peace treaty with their erstwhile combatants. With vigour (and a dash of self-dramatisation), a despondent Marie threw herself into nursing work for the Romanian Red Cross in a nun-like costume of crackling white, in which she was frequently photographed, clutching an armful of Madonna lilies for good measure. The remembrance I keep of those days is of a suffering so great that it almost blinded me, she wrote, yet I was quite calm and continued living and working as though my head had not been torn from my breast.  Marie of Romania had been born Princess Marie Alexandra Victoria of Edinburgh, in 1875, at Eastwell Park in Kent. She was the eldest daughter of Queen Victoria's second son, Alfred, Duke of Edinburgh, known as Affie. Like Lilibet Diana Mountbatten Windsor, the Duke and Duchess of Sussex's newborn, Marie, who had an elder brother (also called Alfred), was the second child of the royal spare. At the time of her birth, she was ninth in line to the throne then occupied by her grandmother Queen Victoria, after her uncle the Prince of Wales (later Edward VII), her five Wales cousins, including the future George V, her father and her brother. No one in 1875 expected Victoria's newest granddaughter to ascend Britain's throne. Like her equivalent in today's royal family, Princess Eugenie  the second child of the Queen's second son  Princess Marie of Edinburgh was born to a life on the royal fringes. The inclusion of Victoria among her names was typical of the Queen Empress's granddaughters  indeed, Victoria insisted on it: all three of Marie's sisters also bore the name. Chief among her parents hopes for their newborn child was marriage to a fellow royal. The Romanian throne was not on their wishlist. At the time of Marie's birth, Romania was still six years away from becoming a kingdom: it was not until 1881 that the German Prince Charles of Hohenzollern-Sigmaringen found himself King Carol I of the new Balkan state. Marie married the heir to the throne in January 1893. A domestic tyrant, until his death in 1914, Carol made the early years of her married life all but unbearable. Instead, Marie's day came in the spring of 1919. Pathe News recorded her arrival in Paris for the peace conference, along with King Albert of Belgium, to urge the claims of their small countries. The odds were stacked against her. After two years of neutrality, Romania had spent a single year fighting with the allies, before an ignominious peace treaty and enforced inaction. That this once superfluous British princess urged her adopted country's claims with remarkable success can be measured from Romania's gains under the Treaty of Versailles. In 1919, Romania doubled in size to become the fifth largest nation in Europe: its new territories included Transylvania and Bessarabia. At a stroke its population rose from eight to eighteen million. One woman's word cannot change the face of such big events, Marie had claimed, but a leading British official described her as the most astonishing propagandist he had ever seen. I know of no one who went away from Paris with more satisfactory annexations than Marie of Romania, one observer noted. To French president Georges Clemenceau, who greeted her on arrival in Paris with the statement that he disliked Romania's prime minister, Marie had replied purringly, Perhaps then you'll find me more agreeable. Clearly he did, and her argument that a disappointed Romania would succumb to revolution and, perhaps, Bolshevism, bringing communism closer to the heart of Europe, proved a powerful one. Marie's case is unusual. As a calling, royalty is concerned with the man or woman in the top job. Second children of monarchs enjoy a brief period in the sun until their elder siblings produce heirs of their own. Over time, the spares inevitably slip down the rankings  unless, like George V or George VI, both second sons, some unexpected event intervenes to change the course of their lives dramatically, in George V's case, the early death of his brother, Prince Albert Victor, the abdication of his elder brother Edward VIII in the case of George VI. For the younger children of the spare, the future is even less likely to yield glittering prizes  or, looked at differently, to impose the relentless burdens of sovereignty. How different has been the life of Lady Sarah Chatto, the second child of George VIs spare, Princess Margaret, from that of her aunt the Queen, the elder child of George V's spare, Albert, Duke of York (later George VI). Historically, royal spares did not even always feel any pressure to marry, let alone provide hypothetical extra heirs to the throne. George II and his formidable queen Caroline of Ansbach greatly preferred their younger son, William Augustus, to their heir, Frederick, whom Caroline described as the most hardened of all liars and prayed that the ground would open this moment and sink the monster to the lowest hole in hell. Even Caroline's vigour could not alter the laws of succession and the royal couple were unable to make William Augustus king in Frederick's place. In April 1736, Frederick married the childish Augusta of Saxe-Gotha, who bore him nine children. With the throne so amply provided for, William Augustus never married. His grandfather, George I, created him Duke of Cumberland at the age of five. History remembers him as Butcher Cumberland, a term coined by Tory opponents after his defeat of Jacobite enemies of the Crown at the Battle of Culloden. His nephew, Frederick's second son, Edward, Duke of York and Albany, shared his lack of urgency over matters matrimonial: he was still unmarried at his death, aged twenty-eight, in 1767. In Edward's place, Frederick's third son, William, found himself the royal spare. Like Marie of Romania, William's younger child had a near collision with eminence. Also called William, the Duke of Gloucester and Edinburgh was Chancellor of the University of Cambridge when, in 1812, a camarilla of Swedish nobles offered him the throne of Sweden following the deposition of King Gustav IV Adolf and the death of the only heir of his successor, Charles XIII. British politicians forced William to decline this handsome offer. Instead he married his cousin, George IIIs fourth daughter Princess Mary. Husband and wife were both forty at the time of their marriage and there were no children. They lived at Bagshot Park, today the home of the Earl and Countess of Wessex. Whatever lies in store for little Lilibet, she is unlikely to find herself Queen of Romania or King of Sweden. Like earlier children of royal spares, however, she may over time enjoy the privilege of privacy denied to those closer to the throne. The Queen by Matthew Dennison (RRP 25). Buy now for 19.99 at books.telegraph.co.uk or call 0844 871 1514