King Charles is expected to say "time and again, our two countries have always found ways to come together" during his address to the US Congress later on Tuesday, in which he will call for "reconciliation and renewal" between the US and UK.
The centrepiece of the second day of the King's state visit comes at a tense time in relations, during which President Donald Trump has repeatedly lambasted Prime Minister Sir Keir Starmer over his response to the Iran war.
Security in Washington DC is extremely tight, with the four-day state visit beginning just two days after a gunman stormed an event attended by the president in the capital.
The King is expected to express sympathy over the attack in his speech.
The official visit, during which the King has been joined by Queen Camilla, is the first since Queen Elizabeth II's in 2007.
In the speech, the King is also expected to say that in times of great international challenges, it is more necessary than ever to stand together to defend democratic values.
The King will urge that beliefs of tolerance, liberty and equality are defended, whether through supporting Nato or protecting Ukraine, according to royal sources.
The King will also tell US lawmakers that the transatlantic alliance is built on 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".
The speech is written on the advice of the government and is expected to last 20 minutes.
It forms part of a packed itinerary that started on Monday, when King Charles and Queen Camilla arrived at Andrews military airbase in Maryland.
On arrival at the airbase, the royal couple were greeted on the runway by the US chief of protocol Monica Crowley and the British ambassador to the US, Sir Christian Turner, among other dignitaries.
They were then given flowers by two children of UK service personnel stationed in the US, before a band performed the British and American national anthems.
The King and Queen went on to be greeted at the White House by President Donald Trump and First Lady Melania Trump.
They met on the South Portico, with the famous building currently under much construction work.
The royal visitors had tea in the Green Room and were brought into the garden to see a newly-expanded beehive, rebuilt in the shape of a miniature White House.
This honey-coated piece of diplomacy was intended to appeal to King Charles, who is an enthusiastic supporter of bee-keeping.
The state visit, carried out on behalf of the UK government, is a soft power attempt to strengthen relations, in a year in which the US is marking its 250th anniversary of independence.
Queen Camilla was wearing a brooch which combines the British and US flags, which was given to the late Queen Elizabeth II by the mayor of New York on a state visit in 1957.
That particular trip had been a diplomatic mission to rebuild the US and UK partnership after the Suez Crisis of 1956, which had put the two countries in dispute over a Middle East war.
King Charles and Queen Camilla then went to a crowded garden party attended by more than 600 people at the UK's embassy in Washington DC. The guests had US and UK connections, with people from politics, science, charities and the military.
It was the traditional garden party fare of sandwiches and scones, but the beef sandwiches had their own diplomatic significance, with the meat coming from the first tariff-free batch of British beef imported after a recently-negotiated deal.
Among the political figures there were former Speaker Nancy Pelosi, Senator Ted Cruz and UK Foreign Secretary Yvette Cooper.
Queen Camilla stopped for some time to talk to a group of women representing organisations campaigning against domestic abuse.
"It's very important to have such advocates and it's a cause very close to her heart," said Sandra Jackson, who had told the Queen about her work for House of Ruth, which supports survivors of domestic violence.
There have been calls for the King and Queen to meet survivors of sex offender Jeffrey Epstein. But that hasn't happened because of concerns about jeopardising the legal processes.
Jackson said she "respected their decision" and welcomed the attention that Queen Camilla was bringing to the subject.
Another campaigner Michelle DeLaune, one of a group talking to the Queen, said it was a sign of progress that such a prominent figure was talking publicly about the issue and raising its significance.
In an interview with the BBC, Epstein survivor Rina Oh called on the King "to hear our stories", adding that it would be "so impactful" if the monarch used his speech to Congress to "acknowledge us".
On Tuesday, after a White House ceremonial military welcome, the King will deliver his speech to both houses of the US Congress, the first monarch since Elizabeth II in 1991 to make such a speech.
It follows a rocky time for US and UK relations, with President Trump critical of UK Prime Minister Sir Keir Starmer over the UK's reluctance to get involved in the conflict in Iran.
There will be a speech too from President Trump at a state dinner in the White House, at an event bringing together politicians and celebrities from the US and the UK.
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