Donald Trump called it “fantastic”, Democrats cheered references to Magna Carta, while the joint session of the US Congress came together in giving it a standing ovation.

King Charles’s address to US lawmakers, while non-political, did not shy away from politics. And, though the president did not take offence – “He made a great speech, I was very jealous” – its pointed mentions of subjects the US president has previously disparaged were not lost on America.

“Beneath King Charles’s jokes and decorum, some subtle rebuttals to Trump”, headlined the Washington Post; “King Charles urges checks on executive power,” said the New York Times.

The fact that Charles’s address was so beautifully crafted and delivered with warmth was almost in itself “an implicit reproach to the president’s own rambling, undisciplined public pronouncements”, said Prof Philip Murphy, the director of history and policy at the University of London.

With the UK and US “special relationship” spectacularly frayed, a lot was riding on this, the most important speech of Charles’s reign to date.

It referenced the importance of Nato, called for continued support for Ukraine, warned of the dangers of isolationism. Mentions of Magna Carta and the principle that executive power is subject to checks and balances received rapturous applause from Democrats. There was a very firm mention of “my prime minister”, Keir Starmer, whom the president has publicly insulted, and the king’s own love of the Royal Navy, which Trump has mocked.

“It’s difficult to imagine he could have gone much further in what he said and what he didn’t say,” said the contemporary political historian Anthony Seldon, praising it as “exceptional”. “He judged it incredibly well: very brave, very smart, very clever.”

Such speeches are teamwork. Good speechwriters know the character of their principals well.

Charles’s key aide, private secretary Sir Clive Alderton, is certainly well placed to give crucial input. Chief point man for dealings with No 10, an experienced diplomat and former ambassador who knows the Foreign Office inside out, he served Charles first as Prince of Wales and now as king, and will have liaised with No 10 and the Foreign Office.

A first draft will have set out government, diplomatic and trade objectives and key points the monarch wished to make. Buckingham Palace, Downing Street and the Foreign Office will have been in lockstep on this – they could not afford not to be.

Input is likely, too, from Charles’s deputy private secretary, Theo Rycroft, who has been in the diplomatic service since 2009, and the king’s director of communications, Tobyn Andreae. Across the pond, the UK ambassador, Christian Turner, and deputy, James Roscoe, a diplomat and former communications secretary to Elizabeth II, will have been discreetly channelling the White House view.

Drafts will have gone back and forth, checking for tone, balance and any late-emerging issues. Work on refining this high-stakes speech continued right up until the morning of delivery to Congress, it is understood.

Throughout the whole process, the king plays a major part in crafting. He makes marks by hand, in red ink, to drafts printed out for him, writing commentary in the margins at considerable length, adding things in and crossing things out. He will also have held meetings with Alderton on the subject.

He is said to be good at jokes, with no need for a joke writer. Certainly there were many, which lightened the tough message. Though which genius came up with the idea of giving the president a bell from HMS Trump, a T-class submarine launched in 1944, is not immediately known.

What resulted was an address that appealed to much of the US political class, if not the Trump administration, according to Murphy.

But its impact should not be judged immediately. “Trump will be back to being Trump on Monday,” said Seldon. “But I wouldn’t judge its impact by what Trump is going to be saying on Monday. I would judge it in the long-term sense, by what the Republicans and the Democrats do; how they view Nato; how they view the Ukraine war; how they view America’s unilateral behaviour; how they view executive power in the person, which is to take more power than the founding fathers intended when they carefully devised the separation of powers.”

“You’ve got to judge it in the long term. He could not have done any more.”

Jonathan Dimbleby, the king’s biographer and friend, felt the address contained both an “implied rebuke to Trumpery” and an “eloquent plea” to remember democracy, the rule of law and checks and balances, as well as stressing that the relationship between Europe and the US was now more important than ever before.

It would have been “honed, honed and honed again”. “There’s nothing in that speech that No 10 would not have given approval to, and large parts of it will have been drafted by No 10 and the Foreign Office, alongside the palace,” he told the BBC’s Newsnight.

Lord Darroch, former UK ambassador to the US, believed it was “brave and bold and actually really excellent”. While there was no direct criticism of the US administration, “it was full of implied rebuke”, he said. Citing references to climate change, and Nato’s support for the US after 9/11, they “were signals of implied rejection of some of the things that Trump says”, he told Newsnight.

The Republican senator and Trump supporter Lindsey Graham said the king had given a “much-needed morale boost” for US politicians.

The Republican congressman and former diplomat Michael Baumgartner thought Charles did “a great job for Great Britain”, specifically picking out the king’s comments on strengthening military capability.

However, somewhat throwing the king’s own words back at him, he said: “What matters is not words but actions. And we’ll need to see action from the British people in terms of fulfilling those promises as well.”

As for whether it would heal the rift between president and prime minister, certainly Baumgartner’s personal feelings towards Starmer appeared unchanged. Referring to him as a “leftist weeny”, he told Newsnight: “I think we saw in the king what we traditionally would like to see from Great Britain.

“Certainly Keir Starmer does not have a lot of respect. He is not perceived as being strong and capable here in the US.”