"It's high risk, high stakes and high opportunity."
That's how a royal source describes next week's state visit by King Charles III and Queen Camilla to the United States.
There is genuine jeopardy in this US trip. It's not just a grip-and-grin photo opportunity, with some warm words and a few celebrities hitting the embassy prosecco.
"The visit comes in the biggest crisis in Anglo-American relations for a century," says Andrew Lownie, author of the best-selling biography of Andrew Mountbatten-Windsor.
"We're dealing with a very unpredictable president," says Lownie. But if anyone can influence Trump, he adds, it's King Charles.
Royal historian Ed Owens says this US state visit takes place at a time of "very unusual" political tensions and will be a "huge global event", where the King will have a chance to champion the "traditional values of democracy, liberty and freedom".
All the ingredients are there for the biggest diplomatic challenge of the King's reign: war, scandal and complicated personalities.
It's taking place against the volatile background of an ugly conflict in Iran and the wider Middle East, with a fragile ceasefire holding for now.
There's the host, President Donald Trump, who recently faced criticism for appearing to depict himself in an AI image as Jesus, in a biblical-style robe with light pouring from his palms.
The president said he had intended to look like a doctor, and later deleted the image. But it's a tricky look for the King, who is head of the Church of England.
While the US president is a self-avowed fan of the royals, he regularly criticises the UK Prime Minister, Sir Keir Starmer, and dismissed UK aircraft carriers as "toys" compared with US equivalents. The King is, of course, the head of the British Armed Forces. And the political relations between the US and the UK, and other Nato allies, are at a perilously low ebb.
There will be nervous officials wondering what would happen if Trump holds forth about Starmer during the visit, even though the itinerary seems designed to avoid any unscripted public chit-chat.
"I don't know how disciplined he will be. The Trump show doesn't get turned off because the King is in town," cautions Max Bergmann, a former senior US State Department adviser under the Obama administration.
And there's personal pressure on the King too. After the scandal involving his brother Andrew Mountbatten-Windsor, it's highly likely that survivors of the sex offender Jeffrey Epstein will be giving interviews complaining that the royal visitors are not meeting them during the visit.
Queen Camilla will however be meeting campaigners against domestic abuse during her engagements.
In terms of challenges on this trip, there's also the 77-year-old King's own health problems, undergoing a packed four-day schedule in the US when he's been living with cancer for more than two years.
The trip will involve set-piece occasions in Washington - addressing Congress and a state dinner; a symbolic visit to the 9/11 Memorial and a glitzy reception in New York; then some natural beauty in a national park in Virginia.
State visits are carried out on behalf of the government, and royal sources recognise this trip is going to be a "delicate balancing act", but say there's a bigger picture of long-term US and UK friendship.
"Yes, we face current challenges, but the visit will also seek to celebrate our nations' historic ties and create the conditions for that partnership to continue long into the future," says a royal source.
Even in the seven months since President Trump's visit to Windsor Castle, the relationship between the UK and US has deteriorated. There have been political rows over the UK staying out of the Iran war, and the US president diminishing the UK armed forces' involvement in Afghanistan - which prompted a personal, if behind the scenes, intervention from the King.
Despite these differences, Trump has been steadfast in his admiration for the monarchy and King Charles.
"I know him well, I've known him for years," the president told the BBC last week about the King. "He's a brave man, and he's a great man."
Whether or not that admiration is reciprocated is hard to say. Because when you ask insiders about how the King sees Trump, they tend to repeat how much Trump admires the King.
Having said that, it's not hard to discern the King's values. He is a flag bearer for post-war liberal democracy, a defender of the rules-based international order. In his tribute to his mother, the late Queen Elizabeth II, he spoke of a better world based on "peace, justice, prosperity and security".
When the King and Trump met last autumn, the King pushed the message about the US needing to support Ukraine, which briefly at least seemed to have prompted action.
The King will be expected by the UK government once again to use that personal respect from Trump as leverage.
"He has that deep well of experience, insight and judgement about how to play it on a personal level," says a royal source.
Trump is a "complete antithesis of the King, but he's a pro and will find areas of common interest", says Lownie of the King's approach.
The King's biggest diplomatic moment will be his speech to both houses of the US Congress on Tuesday. He has to sweeten up the president, while at the same time keep enough savoury moments to defend his own values and to promote the interests of the UK government.
Like a diplomatic training camp, the King will be currently prepping for the trip, going through embassy briefs, reading histories and biographies, getting all the details of his engagements. The flight to the US on Monday will be used for more last-minute preparation.
Every nuance in his speech will be pored over by the Foreign Office. It will want to push buttons on supporting Nato, protecting Ukraine and signing UK-US trade agreements, probably washed down with wartime memories, references to Trump's Scottish mother and jokes about George III losing his colony.
It's the first such speech to Congress from a UK monarch since that of his mother, Queen Elizabeth II, in 1991.
Showing more relaxed times, her speech began with the joke: "I do hope you can see me today". Because at the previous engagement her face had been hidden behind tall microphones, in a moment dubbed "the talking hat".
Her speech defending consensus politics would now sound like a rebuke to the wave of populism - and sits in contrast to Trump's threat to Iran that a "whole civilisation will die tonight".
"Some people believe that power grows from the barrel of a gun," she told Congress. "So it can. But history shows that it never grows well, nor for very long. Force, in the end, is sterile. We have gone a better way. Our societies rest on mutual agreement, on contract, and on consensus."
When King Charles speaks, historian Owens says that the "elephant in the room" will be the Epstein scandal, with members of Congress having called on Mountbatten-Windsor to testify and for the King to meet Epstein survivors.
Sky Roberts, the brother of the late Virginia Giuffre, who claimed she was sexually assaulted by Mountbatten-Windsor three times after being trafficked by Epstein, told the BBC's Newsnight programme he needed the King "to stand up and show his unity with survivors".
Roberts said he and his wife Amanda wanted to meet the King for 10 minutes, and would be "lobbying" for it by going to places they expected him to visit.
"This is a very big deal here in the United States, and it's a very big deal to survivors around the world that he set a precedent" for other world leaders to follow, Sky Roberts said.
Giuffre's sister-in-law, Amanda Roberts, said they did not have to go "deep into conversation" given the potential complications of impeding investigations.
"It's an olive branch that we're looking for. It's the symbolism of it. Acknowledgement, shaking the hand and looking us in the face and saying, 'I will continue on my promise to honour a fair trial. I will support the investigations. And I'm sorry that all these survivors have waited so long for justice.' I think it's that simple."
Mountbatten-Windsor, who has denied all claims against him, reached an out-of-court settlement with Giuffre in 2022 which contained no admission of liability or apology.
Trump might also not want any reminders of the Epstein scandal during the visit.
Lownie thinks there could be heckles or protests along the way about Epstein, but the King has the capacity to reach out to the US public and to revive a deeper partnership. "The roots of the tree are still there," he says of the UK and US alliance.
Royal visits are important in leaving an impression. Queen Elizabeth II's easy relationship with Ronald Reagan seemed to be summed up in pictures of them riding together in 1982. Or there was the diplomatic choreography of Princess Diana and John Travolta dancing together in 1985.
Lancaster University researcher Francesca Jackson has been trawling through the archives to see the impact of such state visits.
The late Queen's speech to both houses of Congress in 1991 was the first because her father George VI in 1939 had refused to make a speech, worrying about how his stammer might be perceived.
King Charles's visit will mark the 250th anniversary of US independence. The late Queen was there for the 200th in 1976, but Jackson's research suggests that the UK ambassador in Washington was among those opposed to her attending.
Showing how sensitivities have changed, the trip was in wake of the Watergate scandal and Richard Nixon's resignation in 1974, and the ambassador warned of "a very real chance not only of embarrassment… but of impairment to the dignity of the monarchy".
There have also been invitations that have been regretted. When the Romanian dictator Nicolae Ceauşescu came to the UK in 1978, a memo from then-Foreign Secretary David Owen asked: "Who agreed to this visit? Did I? If I did, I regret it."
Bergmann suggests next week's state visit can't escape the reality that the special relationship is in difficulty.
"Frankly, it's a situation that is really fraught," he says.
The UK has been attempting to "act as a bridge between the US and Europe, trying to preserve Nato, sanding down the edges of the Trump administration, but it hasn't quite worked", says Bergmann.
Shannon Felton Spence, a director at the Harvard Kennedy School, helped to organise the 2015 visit by the then Prince Charles to the US.
She thinks next week's trip would have been even more "whizz-bang" if the Prince and Princess of Wales had come too, but emphasises how much the royals can influence opinion in the US.
The royals are "the UK's number one soft power tool and nowhere is that more evident than in the US", says Spence.
That's even more the case with Trump, when "the president respects the King, he's in awe of the monarchy".
With the political relationship in such a bad shape in recent months, she says this long-planned visit could prove to be a lucky break.
"This couldn't have come at a better moment for the UK. They're playing exactly the right card, at a time when they didn't even realise they'd be needing to play it," says Spence.
Additional reporting by Toby Mann, BBC News
