It's not hard to imagine the confusion of emotions that will be running through Prince William's mind today: sadness and concern about his father's illness, but also confusion about the enormous challenge he now faces and the inevitable sacrifices that will be made. that he has to bring.
The parallels to those his beloved grandmother faced more than 70 years ago are striking. The then Princess Elizabeth was 25 years old and a young mother when she unexpectedly succeeded her father as monarch.
She and Prince Philip had every reason to hope that they still had a good ten years of family life ahead of them before the responsibilities of the throne intervened.
William is 16 years older than his grandmother at this crucial moment and his children are older, but his expectations for the future would not have been lower.
And even if he doesn't become king soon, his life will change in every other way.
Old Guard: Charles and Camilla lead William, Kate, Harry and Meghan at the Commonwealth Service in London in 2020
Prince Harry and William are believed to have not spoken for many months as their relationship was apparently disrupted by Harry's memoir, Spare
Following news of King Charles' cancer diagnosis, it is unclear how much of his father's appointments will now pass to the prince
Suddenly, all of his and Kate's carefully thought-out plans, with the well-being of their children at the forefront, come into jeopardy. What makes the uncertainty even greater is the intransigence of the king's condition.
Life in the royal family's fishbowl has never felt so lonely or so dangerous. For William, the pressure is twice as great because he has to deal with the additional burden of his father's cancer treatment alone.
Kate was released from hospital where she underwent abdominal surgery just eight days ago and is not expected to resume public duties until Easter. William himself is due to return to work tomorrow after canceling all commitments to look after George, Charlotte and Louis.
It's not clear how much of his father's schedule will now pass to the prince. But the news that he will be taking over his post brings with it an additional distraction – Prince Harry's decision to throw himself into his father's side.
Although it was inconceivable that Harry would not want to fly to Britain once the announcements about Charles were made public, his presence will serve as a reminder of how fractured the royal family has become since Queen Elizabeth's death in 2022.
It will be the first time he has seen his father since the coronation in May last year. To the relief of courtiers, the Duke of Sussex will apparently be making the trip without his wife Meghan. The question is: Will he see William too?
The brothers are believed to have not spoken for many months as their relationship was apparently disrupted by Harry's memoir Spare. In the book, published a year ago, Harry accused William of physically attacking him and Kate of being cold towards Meghan.
As he grapples with his new responsibilities, William's thoughts will almost certainly turn to how his grandmother would have handled such a princely idea.
King Charles's diagnosis also means that all of William and Kate's carefully thought-out plans, which had the welfare of their children at the forefront, now suddenly appear at risk
The Duke of Sussex is believed to be traveling to visit his father without his wife Meghan
Perhaps he also draws some strength from the dignity with which the late queen faced her fate all those decades ago. She did this from a position of strength, of course.
When she came to the throne in 1952, public affection for her father, King George VI, was high for the way he led the nation through the abdication crisis and the dark days of World War II. William comes to the fore when the monarchy is in crisis.
It hasn't even been two years since he and Kate made the adjustment to their lives that he hoped would pave the way for a new royal lifestyle.
Their move from London to Windsor was not only about allowing all three children to attend school on the same day, but also ensuring them a level of privacy that was never quite possible during their time at Kensington Palace.
But the couple's elevation to the status of Prince and Princess of Wales has put this rural idyll to a severe test. “They have a very comfortable life down there at Adelaide Cottage in Windsor Great Park, but it needs a lot of moving parts to work well,” says a friend.
Their employees are still based in London and meetings often have to take place at Windsor Castle.
The contrast between father and son as Princes of Wales could not be greater. While Charles – like William – is passionate about what he believes in, he was also a lobbyist by nature. It's not William. He has his core interests – homelessness, mental health and conservation – but doesn't branch out as much as his father once did. For example, William is not interested in art.
But at least for now, William has to cover a lot more ground than he would normally like. He will almost certainly take on his father's role, welcoming arriving and departing foreign diplomats.
How will he cope with this? And more importantly, what does this change in status mean for his relationship with Harry?
Certainly, my friends say, William has what it takes to be dynamic, and some believe that taking on this additional responsibility could shape him, raising both his profile and his visibility.
First of all, William enjoys a quality that his father had to wait years for: public affection. From Princess Diana, William inherited a natural warmth and confidence that is complemented by Kate's easy-going approach.
Even when they were accused of acting “tone deaf” during their Caribbean trip to Belize, Jamaica and the Bahamas, the criticism of the couple soon faded.
Unlike Charles, who was aware of his future from a young age – he was heir to the throne at the age of three – William was protected from his fate for years.
“He really had no idea until he started school,” says a former aide. While he was at Eton, the Queen began teaching him the traditions and history of the monarchy over tea in Windsor.
Still, he delayed the inevitable as long as he could. After studying and serving in the military, he became a search and rescue pilot for the RAF and later moved into a civilian role with the air rescue service.
He was so different from the king in his approach to fatherhood. While Charles fit the arrival of his sons into his notoriously busy schedule, William put family life first.
This is certainly the influence of Kate's family. With the Middletons, William discovered how enriching family life can be.
He hopes that in time his father's recovery will allow a partial return to those days of domestic bliss that many say will make him a modern and well-rounded ruler.
However, there are two family issues that may influence this assessment: Andrew and Harry. Regarding Prince Andrew, he has strongly supported his father's position that the Duke of York cannot return to his frontline royal duties.
But as far as Harry is concerned, his view differs from that of the king. While Charles is torn by the conflict between his sons and longs for rapprochement, William is convinced that trust, the foundation of every relationship, has been completely destroyed.
Harry's arrival will test William's resolve. Will he agree to see him and, over time, agree to a rapprochement, as his father surely desires? Or will the status quo remain?
Charles' illness and recovery could determine this outcome.