London - Perhaps it was too much to expect two very different boys with two very different wives to forge a lifelong alliance. And perhaps it is too easy to blame American Meghan for heading off with Harry away from palace life into the sunset.
And yet it seems just a moment ago that Harry was as close to Kate as he was to William, a warm-hearted trio delighting the world.
The two princes are, of course, hardly the first husbands whose close friendship was split by wives who simply didn’t get on.
The tragedy is that before Meghan came along, Harry was the very essence of a popular 21st-century prince. He was what every romantic expected a dutiful young royal to be - not only good-looking and a brave soldier, but someone who could laugh easily at himself. Indeed, laughter accompanied him everywhere he went.
Meghan’s arrival in his life changed so much of that. But was this her fault, or was it down to Harry’s uncertainty in how to bring a fragrant Hollywood figure into the restrictive and highly controlled world of royal life?
Meghan, remember, had become accustomed to floating her views in public, on everything from climate change and human rights to clean water and gender equality. This became clear on one occasion when the "fab four" of William, Kate, Meghan and Harry were on stage promoting their Heads Together mental welfare charity, and the Duchess of Sussex naturally dominated the microphone.
Even then, some detected an air of discomfort about William and Kate. Here was a young woman making it plain that her new royal status was something to be used so that her voice could be heard - an unknown philosophy in royal life.
At that moment the silent Kate beside her looked almost dull. In truth, all Kate was doing was following a protocol that has kept the royal family in business for generations.
Ironically, there had been a time when Harry envied his elder brother for having the good fortune to meet and to marry such a wonderful girl as Kate Middleton.
That partly explains why the world was so delighted when he found love with Meghan. After all, when Harry first introduced his bride-to-be, they still looked like the most attractive and popular young couple in the country.
What people especially loved was that while William married a woman whose great-grandfather was a coalminer, Harry ended up with a woman descended from slaves. There was no clearer proof that the Royal Family reflected the rich diversity of modern Britain.
Yet there was already a tiny crack in the brotherly relationship. William is understood to have questioned his younger brother about the hastiness of his engagement to the television actress he’d known only for months. He and Kate were together eight years before he put a ring on her finger - a state of affairs which, it must be said, also attracted widespread comment.
But William has long been the wiser and more cautious brother. He heeded his mother Princess Diana’s advice to marry a girl he knew well. Ruefully, she explained to him that when she had married his father, Prince Charles, ‘we hardly knew each other’.
As usual, William was looking out for his kid brother, just as he always did at school and during the difficult time after Diana’s death. But Harry, besotted with worldly divorcee Meghan, didn’t welcome this guidance, and the brothers’ relationship soured when words were exchanged between Kate and bride-to-be Meghan over the dress Princess Charlotte, now four, would wear as a bridesmaid.
At one stage Kate was said to have been reduced to tears. But no one attached too much significance to the kind of small family tiff hardly unknown during nuptial preparations.
Further tension in the lead-up to the event threatened to spill over after Meghan insisted she be allowed to choose a tiara once worn by Diana. This time it was the Queen who said no and instead allowed Meghan to wear the diamond lozenge bandeau made for Queen Mary in 1932.
Meghan accepted the Queen’s decision. But it seriously upset Harry and provoked his emotional outburst "what Meghan wants, Meghan gets". As for palace aides, the general feeling was that here was a mature woman of the world who had comfortably fitted into a new life in Canada - where she made her home for seven years while filming TV series Suits. Surely she would have as little difficulty adjusting to royal life as a young Kate did?
But one was happily restrained by the knowledge that she is destined to be Queen; the other uncomfortable unless she is permitted to speak about what is on her mind. As one royal aide observes cautiously of the royal sisters-in-law: ‘They are simply not natural chums.’
Even so, palace life might have jogged along well enough had Harry and Meghan not taken the surprising decision to remove themselves from the centre of public life in London and decamp to Windsor. Harry’s greatest dream was about to be realised: Meghan was pregnant.
Some might feel she wanted to move away so she and Harry could have a more peaceful time. But in many ways this was a precursor of things to come. For who would have imagined the public’s love for a new baby, especially a royal baby, would have been so defiantly frustrated?
No reason has ever been given for the couple’s secrecy in refusing to say where baby Archie was to be born. Nor has an explanation been supplied for why the couple chose not to say when she was in labour.
Equally confusing is why they refused to disclose who were the doctors attending Meghan. These were crucial days for Meghan and Harry, when the public should have fallen in love with them and their baby. Instead, they induced a wave of exasperation from people who considered the arrival of the seventh in line to the throne as a matter of public rejoicing.
Inexplicably, the couple chose the same level of secrecy over the christening, declining to say who were Archie’s godparents. No royal couple in history had ever behaved like this before. But then, why on earth would they?