It was billed as the biggest day of the trip and it did not disappoint. And it was surely the moment that helped to turn the page on a growing rift between the UK and America.
From President Trump's speech full of praise for the UK and its royal family, delivered during a spectacular display of pomp and pageantry on the White House lawn, to the King's barnstorming, historic address to Congress and both men's warm words at the candlelit state dinner, this was soft diplomacy in overdrive.
And with one notable exception, everything ran like clockwork. My day had begun at 6am with an early start to get through White House security and into the press briefing room to await our escort on the South Lawn. I never forget what a privilege it is to witness historic moments, and taking in the surroundings of arguably the most famous address on the planet is certainly up there.
© Getty ImagesThe last time I was there with King Charles was in 2015, when, as Prince of Wales, he held talks in the Oval Office with President Barack Obama and Vice President Joe Biden. Eleven years on and yesterday he was back in that most famous of rooms, with a different President, of course. But his return, this time as monarch, showed the extraordinary continuity our royal family provides.
In the 56 years since Charles first visited, the US head of state has changed 10 times, but the UK's, only once. It's that longevity that has helped build an enduring link between our two countries despite the ebbs and flows of day-to-day politics. When Donald Trump addressed guests at a ceremonial welcome for the King and Queen yesterday, he could not have spoken more highly of the royal family.
From his late mother's love of Queen Elizabeth II – and her admiration for the "cute" young Prince Charles – "My mother had a crush on Charles. Can you believe it?" – to his own enthusiasm for the King – "a man of class" and "a fantastic person" – there was no doubting the might of His Majesty's soft diplomatic power.
My mother had a crush on Charles. Can you believe it?
© Getty ImagesThat power was highly visible again in Congress, when the King and Queen were welcomed with a four-minute standing ovation by both Houses. Standing in the same place from which his mother, Queen Elizabeth II, gave her own address in 1991, he didn't shy away from topics where there have been differences of opinion between both countries, including the need for "unyielding resolve" to defend Ukraine and the role of NATO.
See more from day two of the King's state visit below...
The King also raised the need to protect the natural world, receiving one of his many ovations for saying: "We ignore at our peril the fact that these natural systems – in other words, Nature's own economy – provide the foundation for our prosperity and our national security."
Although President Trump, a climate sceptic, was not present in the chamber because of protocol, he was watching from the White House and may well have taken note. Not everyone in the chamber agreed with the King, of course, but from where I was sitting, it felt as though he had his bipartisan audience in the palm of his hand.
One palace insider jokingly dubbed the warm reaction to the speech "Clapital Hill". The King, I'm told, was "deeply honoured" to have been invited and "greatly touched by the warmth and generosity of the response he received." Immediately afterwards, we were whisked back to our hotel for a 20-minute "break" to change into black tie and gowns for the evening's events.
But the King's charm offensive – and considerable stamina – was unstinting. He may have been short on time, but before changing for the evening's events, he managed to squeeze in a meeting with tech leaders back at Blair House, where he and the Queen are staying. Amazon CEO Jeff Bezos and outgoing Apple CEO Tim Cook were also guests at the state dinner.
© Getty ImagesQueen Camilla deserves a mention too. She and the First Lady appeared to get on famously, giggling together and clearly enjoying the spectacle in the morning, and beaming for the cameras for a full minute ahead of the dinner – in a complementary shade of pink to Melania's Dior gown.
Inside, the arrangements had more than a touch of a state banquet back at Windsor Castle – with gorgeous floral arrangements that filled the air with scent and elegant place settings. There was even a harpist playing musical numbers including Coldplay hits in a corner.
Then, about 90 minutes after their car had pulled up outside, the King and Queen were announced and they followed their hosts into the East Room. In his remarks, Mr Trump once again praised the shared history of the UK and America, but there was a sharp intake of breath from some in the room when he veered onto the topic of the current conflict in Iran.
© AFP via Getty ImagesIn a moment that will concern Palace officials, the President declared that, "Charles agrees with me, even more than I do, we're never going to let that opponent have a nuclear weapon." The King, as a constitutional monarch, must be above politics. There will be more to say about this in the morning, but it's a reminder of the real jeopardy that surrounded this trip and the risk taken in proceeding with it as planned.
For now, I'll focus instead on the King's own remarks. Once again, he delivered a masterclass. Lighter in tone than his earlier address, he peppered his speech with jokes and made a personal gift to the President of the original bell from the Royal Navy submarine HMS Trump, which fought in the Battle of the Pacific in the Second World War.
A thoughtful gift, delivered with classic King Charles humour: "If you ever need to get hold of us…just give us a ring." The President leapt to his feet in clear approval.
© AFP via Getty ImagesCharles also joked that the dinner was "a considerable improvement on the Boston Tea Party" and, in a diplomatic sleight of hand that will have been appreciated north of the border, gently reminded Mr Trump that he is head of state in Canada. He said of this year's football World Cup, which takes place across the US, Canada and Mexico, "In one sense, we are joint hosts."
It was another superb effort from the King, who is clearly giving this visit his all. As for us journalists, it's been a long day, but a thrill to witness history being made up close.
When I close my laptop in a moment, it will be just five hours until I leave the hotel again, this time New York bound. I might need a coffee or several before my next instalment.
I will be covering it for Hello throughout the week and sharing my analysis and opinions with VIP subscribers
