William is known for his stubbornness, but he would never have dared invite them without the approval of the Queen, who would have understood its significance.
Indeed, she is very fond of Kate and comfortable with her unassuming manner, in much the way she enjoys being with Prince Edward's wife, Sophie, whose father worked for a tyre firm.
But in the wake of this highly significant family tableau at the Royal Military Academy, and just when it was assumed that 'Princess Kate' was a done deal, two things happened.
One was that Kate, inadvertently, may have over-played her hand.
So sure of William, she confided to friends of a possible imminent engagement announcement.
The other was that he suddenly got cold feet.
One of William's friends explained: "Suddenly, the enormity of what he was drifting into hit him - and hit him hard. It sort of woke him up to what was happening, and seemed to send him off the rails a bit."
This was the period when William was seen out partying and enjoying himself without Kate, and he allowed himself to be photographed with other girls.
William also confided his uncertainty to the Queen and Prince Philip. He wasn't sure that he "loved Kate enough".
The word "enough assumes real significance here, because he didn't say that he did not love Kate.
It was the degree of love he queried - rather poignant, coming from the son of a man who astonished the world by choosing the very day of his own engagement to Diana Spencer to pose the philosophical query: "Whatever love means...?"
When Kate learned of his uncertainty, her inevitable next step was to demand from William some kind of assurance about the future.
He couldn't give it. So they split.
Seeing them together now - or watching them, as fellow diners did, at Locanda Ottoemezzo, a popular neighbourhood restaurant in a quiet Kensington street just the other night - all that turmoil and uncertainty seems to be unreal, and very far away.
So is this the real thing?
When William was not gently cradling her head in his hands, they were holding hands.
And this, remember, is a couple who have been sharing their lives for more than four years. Not a new love, then, but perhaps a love which, having been tested apart, has found a new intensity.
Is William certain now? It looks that way.
But he is 25 and he has said, more than once, that he does not intend to marry before he is 30.
Presumably the next step will be for the couple to want to set up home together.
Edward and Sophie lived together (under the Queen's roof at Buckingham Palace), but for a future King to do so would require rather more thought and sensitive planning.
And don't underestimate the unique ability of most cackhanded royal advisers to mishandle matters.
Despite all the portents, William is still young enough to be revisited by an attack of cold feet.
He may have his mother's looks and be a popular prince, but if another break-up occurred, millions of women would see it as a case of "like father, like son".
How ironic it would be if he ended up taking the same kind of flak that rained on Prince Charles over the way he treated Diana.
"Oh please, not that," murmurs one courtier.
"He must never be like his father."
