The Palace acknowledges this was a trip with "challenge and risk".
But as they boarded the flight home, the view from the royal plane was clear: "This was a week well spent."
It's certainly been a great piece of PR for the King.
It's unlikely, though, to reassure those who thought a state visit to the US was a bad idea. Questioning the sense of sending the King at a time of such strained relations.
So has the trip turned the tide when it comes to Anglo-American relations? "I wouldn't say we're there to change the relationship," said a senior Palace aide.
But there is a feeling that the King has a Midas touch with Donald Trump, reaching parts of the president that British politicians cannot.
The Palace believes he helped the government "get the relationship perhaps more widely understood by a wider number of people without some of the noise of current affairs".
What benefits it actually brings Britain is still to be seen, minus the lifting of whiskey tariffs.
Observing Trump at the White House, it's obvious how much he respects and likes the King. The same cannot be said for the prime minister, who's repeatedly been on the wrong end of Trump's tirades.
"It's not a competition between the King and the government," said the aide.
But perhaps the PM's office will be watching and wondering.
We're told the King and Trump "get on very well" as do the Queen and First Lady.
A surprising pairing? "Given some of the issues that have presented themselves in the bilateral relationship, you'd think it might all be a bit tricky. But far from it," said the aide.
As for the speech to Congress, touching on the testier sides of the UK-US alliance, including NATO and the war in Ukraine, these were deliberate inclusions and issues the King, we're told, wants to speak out about.
"It's a measure of how much he personally cares [that] these were all themes," according to the aide.
Read more:
Why King's US visit probably won't have been worth it
Best pictures from the King and Queen's state visit
Some say it put the King perilously close to politics.
"First, what the King says will always be guided by the truth. Two, it will be guided by conscience. And three, they're all observable facts. Everything that is in that speech is an observable fact," the aide said.
How did the King feel things went? As he arrives home, his mind is already moving on.
The senior aide said: "He's not a man to dwell long on what some may consider yesterday's successes."
The King's four-day visit was followed by a trip to Bermuda to celebrate its culture, people and achievements, which ended on Saturday.
In a farewell message to the British overseas territory, Charles said in a social media post that "the fond memories" would "last a lifetime".
(c) Sky News 2026: King has Midas touch with Trump, but benefits for Britain remain to be seen


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