One of the “what if?” questions often posed by historians asks what would have happened if Henry, Prince of Wales, had not died in 1612, at the age of 18. Sporty and dashing, he was everything that his weedy younger brother, Charles – the future Charles I – was not. Surely he would have been a popular, charismatic king, incapable of leading the country into a civil war?

But here’s another question, which is almost never asked. When the sickly young Prince Charles did succeed his brother as heir apparent, he was not expected to live long; and if he had died, their sister Elizabeth would have been next in line. So what sort of sovereign monarch would she have become? Could she have plunged the country into war? Or would she have presided over another golden ­Elizabethan age?

After reading this major new biography of Elizabeth Stuart, I’m still slightly unsure of the answer (though these “what ifs” are, of course, a mug’s game, as there are always too many other variables). Yes, she was charming and clever, and the public would have adored her. They already did when, less than two months after the death of her beloved brother Henry, she was married off, aged only 16, to a German prince, Frederick of the Palatinate. All London turned out to see the fireworks display, involving 36 ships and 1,000 soldiers; adoring poems poured from the presses, and she quickly acquired the nickname “Queen of Hearts”.

But she could also be extraordinarily stubborn and unyielding. She had a strict view of her rights as a royal princess, causing ructions at her husband’s court in Heidelberg when she insisted that she had royal status and he did not. The ­protocol problem may have been solved when, in 1619, he rashly accepted the crown of Bohemia; but a thousand much worse problems flowed from that, as they were driven out of Prague by a Habsburg army after just one year, earning the derisory titles “the Winter King” and “the Winter Queen”.

The marriage had been a little rocky to start with; but now sheer adversity – of which there was more to come, as they lost all their territories and were driven into exile in Holland – bound them together. Elizabeth became a fierce champion of her doomed husband, as well as a protective mother of their many children. And while much of Germany was trashed by the Thirty Years’ War, which Frederick had helped to set off, she worked non-stop at fundraising and political string-pulling, always convinced that the next battle or campaign would restore them to all their rights. (It never did.)

Poor Elizabeth. You cannot help sympathising with her, as each of her allies dropped out in turn, and each of her generals either blundered on the battlefield or died – including her most devoted supporter, Prince Christian of Anhalt, who, when he lost a hand in her service, perkily adopted the motto “altera restat”, “I’ve still got the other one”.