The King will attempt to calm troubled waters tomorrow in a historic speech to Congress telling politicians, 'time and again, our two countries have always found ways to come together'.
He is also set to make a pointed rejoinder to President Trump's scathing dismissal of the UK's armed forces, particularly the Royal Navy, gently pointing out his own proud history in the service.
In one of his recent broadsides, sparked by what he sees as British Prime Minister Sir Keir Starmer's failure to support his war with Iran, the American leader described UK assets as 'toys', adding: 'You don't even have a navy. You're too old and had aircraft carriers that don't work.'
Royal aides say His Majesty - who is head of the British armed forces - will reference 'with pride' his own service in the Royal Navy, and emphasise how the US and UK have stood together through moments that have defined our shared history, observing how 'our defence, intelligence and security ties are measured not in years but in decades'.
In doing so, he will reference current topical matters including NATO - which Mr Trump has threatened to pull out of - AUKUS, the Middle East and Ukraine.
The King will become only the second British sovereign to address a joint meeting of Congress, the first being his mother, Queen Elizabeth II, in 1991.
President Donald Trump and first lady Melania Trump greet King Charles III and Queen Camilla as they arrive at the White House today
Pictured: King Charles III with US President Donald Trump at Windsor Castle on September 18, 2025
King Charles III and Queen Camilla are greeted by US Protocol Chief Monica Crowley (R) as they arrive on day one of their State Visit to the United States today
His 20-minute address is written on the advice of Government to support their strategic priorities, but the language and tone will be his own.
The overall message of the speech will be that while we live in times of great challenge, nationally and internationally, by defending and reaffirming our shared democratic values - values which have been woven deep into the fabric of both our nations throughout history - we can promote security and prosperity now and for the future, not just for our two countries, but for the whole world.
In his opening remarks, Charles will also make brief reference to the events of Saturday evening, in a spirit of solidarity, sympathy and support, as he brings 'the highest regard and friendship of the British people to the people of the United States' in this landmark anniversary year.
Turning to the main theme, he will reflect that while the UK and US have not always agreed on all matters over the past 250 years, the foundations of our 'democratic, legal and social traditions' - stretching all the way back to the Magna Carta - are such that 'time and again, our two countries have always found ways to come together'.
When they do so, he will say, the results are not just of national, but global benefit.
In addressing these values, the King will speak of his own faith and his belief that in the hearts of our two nations lies 'a generosity of spirit and a duty to foster compassion, to promote peace, to deepen mutual understanding and to value people of all faiths and none'.
Such values, he will say, need defending.
He plans to reflect on how defending our common ideals is 'crucial for liberty and equality', as is the rule of law which lies at the heart of our democratic traditions and of our shared economic prosperity.
President Donald Trump bids farewell to King Charles III at Windsor Castle during the State visit by the President of the United States of America on September 18, 2025
Looking to the future, His Majesty will reference the importance of the trade and technology deals that will play a vital part the continued economic partnership of the two counties.
He will also celebrate America's many natural wonders, and stress how important it is to defend and protect 'Nature's own economy', too.
How well those comments will sit with President Trump, a notable climate change denier who has ripped up many of his country's green initiatives, remains to be seen.
However the two men have cordially discussed the issue before.
The King will conclude by noting that the story of our two nations over the last 250 years is one of 'reconciliation and renewal', giving rise to 'one of the greatest alliances in human history'.
It is an alliance which His Majesty hopes and prays will continue to protect and celebrate the values he holds dear, long into the future.
Later in the day the King will give another speech at a glittering state dinner at the White House, where President Trump will also speak.