Of the many jokes cracked by King Charles during his visit to Washington, the one recalling the definitive 18th-century Anglo-French contest for dominion over the New World was the most pointed. Speaking at a state banquet in the White House, Charles turned to Donald Trump and said: “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!”

Did Trump get it? Who knows? Broadly speaking, history, even their own, is not most Americans’ favourite subject. A forward-looking people, they do not dwell on the past, nor hanker after the illusory felicities of former glories. While generations of Britons still wallow in nostalgia for Spitfires, Churchill and Vera Lynn (and beating the French), Americans typically seek new metaphorical mountains to climb. Theirs is a positive outlook, on the whole. Except, under Trump, it has twisted into a revived, ugly version of US “manifest destiny” imperialism.

In his quiet, understated way, Charles had a lot to say about all that. Addressing Congress, he did not give Trump the serious tongue-lashing many in Britain (myself included) had been hoping for. Given the constitutional and political constraints, it was a ballsy performance nonetheless. Charles may have succeeded in temporarily easing US-UK frictions. But his bigger achievement was to remind Americans, ever so gently, of who they are, where they come from, and how very much better they could and should be doing.

To put it mildly, the US, led by its manic president and the Republican party, has been acting out of character for a while now. Charles’s proffered antidote was calm, balm – and perspective. He supplied a mature, knowing lens through which to view, rise above and look beyond the trials and tribulations of the Trump era. He articulated a belief in the US that Americans are in danger of losing. He spoke of unity as an essential condition of success. He stressed that what the US does matters everywhere. Charles’s subtle, much-needed history lesson may have done more than Trump ever has to make the US feel great again.

The reaction of Democrats and many Republicans in a fractured Congress was telling. Again and again, they rose together to applaud the king’s evidently sincere conviction, implicit rather than explicit, that the US will get through this, will come to its senses, will rediscover its principles, will once more aspire to act as a moral force for good – his conviction that the nightmare will end, as, history shows, nightmares always do.

Remember Magna Carta? That English charter of 1215 curbing the power of kings was a crib sheet for the US’s founding fathers and had been cited at least 160 times in US supreme court cases, Charles said. It established “the principle that executive power is subject to checks and balances”. Who could miss this real-life king’s deft allusion to the importunities of the overweening pseudo-king in the White House? Democrats certainly didn’t. They stood and cheered.

Remember the 1688 bill of rights, product of the English civil war and the struggle for parliamentary sovereignty? Chunks of that text were lifted verbatim and incorporated in the 1791 US bill of rights, he noted. Here was candid royal backing for those who fear present-day US civil liberties are falling victim to recycled tyranny. Remember 9/11, a quarter of a century on? Nato countries such as Britain certainly do, Charles said. They also remember how they rallied round the US. Unspoken message: value the support and loyalty of the UK and your European allies. And reciprocate. Help Ukraine.

The king’s reminiscences about previous royal tours further served to refresh collective American historical memory – and underscore his theme: that no matter how big or strong, no single country can go it alone for long. Charles’s mother, Elizabeth II, had been a good friend to every president since Eisenhower. Such connections, he suggested, reflected the deep, abiding ties between the two peoples. The US, though a successful, independent nation, remained rooted in Britain and Europe. And, he almost said, don’t you ever forget it!

In a way, it was obvious, hackneyed, even manipulative stuff. But the enthusiastic reaction in Congress and the US media suggested Americans – their national sense of self under daily assault, their fears for the future ever more pronounced, their nerves exhausted and lives disrupted by endless Trump traumas and tantrums – badly needed to hear it. George Canning, Britain’s foreign secretary in 1826, famously “called the New World into existence to redress the balance of the Old”. Through Charles’s reaffirming visit, the “Old World” returned the favour.

It’s true. Politically as well as historically, Trump’s reign has thrown the US radically off-balance. Half the country seems to think it’s at war with an enemy within and ungrateful, rapacious foreign allies. The other half despairs of a president who actively undermines the democratic values and laws rebellious colonists fought to uphold 250 years ago and upon which the US constitution – and US legitimacy in the world – rests. King Charles went to Washington to save Britain’s bacon. Through his example and unassuming advice, he showed the US how to save itself.

Will Americans heed his message? Will they take history’s lessons to heart? Or will it all turn out to be a temporary blip, a fleeting moment of goodwill and good manners, a mere gap in the clouds? No sooner had Charles left Washington than Trump, predictably, began exploiting their private conversations to justify his Iranian inanities.

The Iran war – barely mentioned during this visit for fear of eruptions – is an acid test. If the Trump administration were to adopt Charles’s calm approach, stand back and dispassionately examine the history of this senseless feud, thinking back to the CIA’s anti-democratic 1953 Mossadegh coup, the installation of the Shah’s dictatorship, and the long decades of irrational vilification, mutual ostracism and sanctions that followed the 1979 revolution – including US support for Saddam’s Hussein’s 1980s war of aggression and Israel’s long, lethal shadow war – maybe it would act differently now.

Since he apparently likes the British way of doing things – and in the spirit of Charles’s visit – Trump should follow the UK’s prescriptions, not restart the war. De-escalate, pursue unconditional, good-faith negotiations, and offer an end to sanctions and diplomatic normalisation in return for Iran’s pledge to forgo nuclear weapons development and close down regional proxies. That’s the deal everyone is waiting for. It’s the only one that will stick.

If Trump, taking the long view for once, chose to do it, he could belatedly put the US back on the right side of history. And king or no kings, the world would have reason to celebrate the week Mr Windsor went to Washington.

• Simon Tisdall is a Guardian foreign affairs commentator