There was a whole battalion of lively young men,” recalled Lady Anne Glenconner, whose family were friends and neighbors of King George VI and Queen Elizabeth at Sandringham, their estate in Norfolk. But Princess Elizabeth, the heiress presumptive to the British throne, “realized her destiny and luckily set her heart on Prince Philip at an early age. He was ideal—good-looking and a foreign prince.”

Her choice was in some respects traditional, because the princess and Philip were relatives, but not too close to raise eyebrows. They were third cousins, sharing the same great-great-grandparents, Queen Victoria and Prince Albert. Philip was in fact more royal than Elizabeth, whose mother was mere British nobility (with distant links to English and Scottish kings), while his parents were Princess Alice of Battenberg (a great-grandchild of Queen Victoria) and Prince Andrew of Greece, the descendant of a Danish prince recruited for the Greek throne in the mid-19th century. Elizabeth and Philip were both connected to most of Europe’s reigning families, where consanguinity had been common for centuries. Queen Victoria and her husband had been even closer: first cousins who shared the same grandmother, the Dowager Duchess of Coburg.

In other ways, Philip was an outlier with a decidedly unconventional background. Queen Elizabeth had made no secret of her preference for one of her daughter’s aristocratic English friends from a family similar to her own English-Scottish Strathmores—the future Dukes of Grafton, Rutland, and Buccleuch, or Henry Porchester, the future Earl of Carnarvon. Philip could boast none of their extensive landholdings, and in fact had very little money.

Although he was born on June 10, 1921, on the isle of Corfu, Philip spent scarcely a year in Greece before the entire royal family was expelled in a coup. His parents took him, along with his four older sisters, to Paris, where they lived rent-free in a house owned by wealthy relatives...