For his last trick, the king revealed a bell that hung from the conning tower of a Royal Navy submarine launched from a UK shipyard in 1944. Its name was HMS Trump. “And should you ever need to get hold of us,” Charles III said, “well, just give us a ring.”

The polished brass bell bearing the name “Trump”, presented at Tuesday’s state dinner at the White House, was an ego-flattering masterstroke that will have prompted groans in foreign capitals from Paris to Canberra to Tokyo. How can they ever hope to match that?

But for all the gushing praise on both sides of the Atlantic for Charles’s elegant display of diplomacy on his visit to the US this week, British prime minister Keir Starmer would do well to remember the problem with soft power is it is soft, and can quickly scatter like blossom on the wind. Donald Trump is notorious for blowing hot and cold: while the monarch bathed in the warmth of his anglophilia, citizen Starmer can still expect the cold shoulder.

This was a trip laced with ironies. Back at home, Charles is the ailing head of a tainted family that symbolises class privilege and colonialism and would never be invented today. Yet in the US, the country that unceremoniously kicked out his great-great-great-great-great grandfather 250 years ago, he was hailed as a debonair defender of democracy.

How did he pull it off? Like a rapier wrapped in ermine, Charles managed to tame Trump while rebuking Trumpism. He wrote a love letter to America while eviscerating the “Make America great again” movement. His style appealed to Republicans’ warm, fuzzy feelings about Britain; his content appealed to Democrats anxious about institutions and the rules-based order.

A decade ago Charles’s address to Congress would have seemed boilerplate in its support for the Nato alliance and a western ally’s war against Russian aggression. His remark that Magna Carta has been cited in at least 160 supreme court cases since 1789, “not least as the foundation of the principle that executive power is subject to checks and balances”, would have been one for the history nerds.

It is a sign of how far Washington has sunk that now such comments were seen as positively daring, speaking truth to superpower and impressing on the US body politic of what it has lost. Paradoxically, it took a direct descendant of the tyrannical George III to warn the young nation that it is betraying George Washington.

Jon Meacham, a presidential historian, told the MS Now channel: “It’s sort of like having a headmaster speak to a school. He has come over to remind us of what matters, of what’s important, of what has endured not simply because it is old but because it is true and has been of utility.

“I don’t know whether this smooths over immediate diplomatic relations but I do know that read, not even particularly carefully, here you had a king reminding a democracy and a republic of what matters. He said our words matter; our deeds matter; don’t look inward; remember that there are checks on executive power; note the climate.”

Britain separates his head of state from its political leader; America wraps them all into one. The danger of the latter approach has become all too evident when that person is wannabe emperor. Like a billionaire who feels liberated to talk about inequality and taxes, Charles’s exalted status above the fray seemingly gave him special licence.

Meacham added: “This is a kind of masterclass in how someone not tied to the minute-to-minute political realities of popular politics – this is what this thoughtful man believes is important across the Atlantic. I think it’s a vivid and elegant, in many ways, warning and inspiration as well about not losing ourselves in a populist, isolationist, nationalistic moment.”

Charles’s speechwriter may have calculated, probably rightly, that the political points would have sailed over the head of Trump, who had eulogised the king’s “beautiful accent”, reminisced about his Scottish-born mother having a “crush” on the young prince and indulged disturbing blood-and-soil nationalism, suggesting that the US is defined by Anglo-Saxon cultural and genetic heritage.

But the king’s remarks, leavened with quips and quotations, did provide an ego boost to members of Congress and the supreme court who have been systematically marginalised by Trump’s expansion of executive power. The coded message to them was: you matter too, now get your act together.

Charles’s speech at the state dinner was similarly well received, gently nodding to Trump controversies without appearing to chide him. “You recently commented, Mr President, that if it were not for the United States, European countries would be speaking German. Dare I say that, if it wasn’t for us, you’d be speaking French.”

The king even made a glancing reference to the current rift over the Iran war and the historical echo of the 1956 Suez crisis. Again, the charmed Trump didn’t seem to mind, blanking out the bits he didn’t want to hear.

The king proceeded to New York and an immediate tonal shift. There was no fawning reception from Zohran Mamdani, the Ugandan-born democratic socialist mayor whose father Mahmood has written books including Contemporary Africa and the Legacy of Late Colonialism.

Asked on Wednesday morning what he would say if they were to spend time together, Mamdani said he would probably encourage the king to return the Koh-i-Noor diamond, one of the Tower of London’s crown jewels that was taken from a 10-year-old maharajah in India whose kingdom was seized by the British.

Mamdani’s brief meeting with Charles was courteous enough. But the politically astute mayor had offered a corrective to the excessive adulation that trailed the king and his utterances of “By Jove!” What Trump and Charles’s historically selective speeches failed to mention was the dozen British monarchs who sponsored, supported or profited from Britain’s involvement in slavery.

Nor did they grapple with modern scandals ranging from the treatment of Diana, Princess of Wales and Meghan, Duchess of Sussex to the recent arrest of Charles’s brother, Andrew Mountbatten-Windsor, over his connection to the convicted child sex offender Jeffrey Epstein. Appeals for King Charles and Queen Camilla to meet survivors of Epstein’s abuse went unheeded.

Yet headline writers proclaimed the visit an unmitigated triumph, casting Charles as the comeback king after years in which the monarchy has seemed like a crumbling castle with threadbare carpets and dark secrets. Trump announced on Truth Social that he would be removing tariffs on Scottish whisky in honour of the king and queen.

If anything, Charles might have hurt Starmer rather than helping him by throwing the contrast between them into sharp relief. Trump admires and envies a man who wears a crown and sits on a throne; he is less impressed by a human rights lawyer who got elected to Downing Street. The president told Britain’s Sky News that Charles is “a much different person than your prime minister. Your prime minister has to learn to deal the way he deals and he’ll do a lot better.”

Dealing with Trump has proved exasperatingly impossible for Starmer and many other world leaders. They must still face the unpalatable truth that he is a thin-skinned narcissist with delusions of grandeur, now reportedly comparing himself to Napoleon, Julius Caesar and Alexander the Great.

That is why the trophy from HMS Trump was such diplomatic genius. It will doubtless take pride of place in the Oval Office or the new President Donald J Trump ballroom. Every time a bell rings, an angel gets its Diet Coke.