The King hailed the “priceless and eternal” relationship between Britain and the US during an address to Congress.
The King and Queen arrived in the House to applause lasting several minutes before the speech.
Addressing the special relationship, the King said: “The bond of kinship and identity between America and the United Kingdom is priceless and eternal. It is irreplaceable and unbreakable.’
Donald Trump has frequently attacked Britain and Sir Keir Starmer in recent months, and officials hope the King’s visit will heal divisions between the allies.
The King made a reference to the Iran war, a point of contention between the US president and British prime minister, acknowledging that “we are meeting at times of great uncertainty”.
The King also paid tribute to “victims of some of the ills that, so tragically, exist in both” the US and Britain’s societies, amid pressure to meet with people who had suffered at the hands of Jeffrey Epstein during his visit.
Earlier in the day, Ro Khanna, a senior Democrat suggested the King would allude to victims of the paedophile. The address to Congress was the first by a British monarch since 1991.
Senior Democrat rebukes Trump in reaction to King’s speech
A senior Democrat said the King’s speech should serve as a reminder that “friends and allies matter”, in a pointed rebuke of Donald Trump’s attacks against Nato.
Chuck Schumer, the minority leader of the US Senate, said: “It has been thirty-five years since a British monarch addressed the elected representatives of the American people – the world has changed dramatically, but in many ways, the same tests remain before us.
“The visit of King Charles III should serve as a reminder to the President of the United States and to every elected official in this country: friends and allies matter.”