King Charles III used a rare address to a joint meeting of the US Congress on Tuesday to urge Britain and the United States to deepen co-operation on defence, democracy and the environment, saying the transatlantic partnership was now “more important” than ever because the challenges facing the world were “too great for any one nation to bear alone”.

Speaking to a packed chamber on the second day of his state visit to Washington, the King blended history, humour and diplomatic signalling in a speech that drew repeated applause and standing ovations from lawmakers. He became only the second British monarch to address Congress, following Queen Elizabeth II’s appearance in the same chamber in 1991.

“We are meeting at times of great uncertainty,” Charles said, referring to conflicts in Europe and the Middle East and to recent political violence in Washington. “Whatever our differences, whatever disagreements we may have, we stand united in our commitment to uphold democracy, to protect all our people from harm.”

He said the UK-US relationship remained indispensable but warned that it could not rely on legacy alone.

“Our alliance cannot rest on past achievements or assume that foundational principles simply endure,” he told members of the House and Senate. “The challenges we face are too great for any one nation to bear alone.”

The King repeatedly returned to the idea that the two countries’ shared history had to be matched by present-day action. Recalling his late mother’s address to Congress 35 years ago, he said he had come to express “the highest regard and friendship of the British people to the people of the United States”.

On security, Charles made one of the clearest policy notes of the speech, calling for continued resolve in support of Ukraine. He said the same determination that had bound Britain and America in past conflicts was now needed “for the defence of Ukraine and her most courageous people – in order to secure a truly just and lasting peace”.

He also highlighted Nato and the military ties between London and Washington, saying British and American “defence, intelligence and security ties are hardwired together through relationships measured not in years, but in decades”. Referring to the alliance’s invocation of Article 5 after the attacks of 11 September 2001, he said: “We stood with you then and we stand with you now.”

Drawing on the Anglo-American legal tradition, the King said that in both countries executive authority must be constrained by law. A line stating that executive power was subject to “checks and balances” drew one of the strongest reactions in the chamber, with applause beginning on the Democratic side before spreading more widely.

Charles said the “shared values of the rule of law” had created the conditions for centuries of prosperity in both nations. He pointed to ongoing economic and technology ties, including work on artificial intelligence, medicines and nuclear technology, as evidence that the relationship extended well beyond symbolism.

The final section of the speech turned to one of the King’s longstanding causes: the environment. Warning of the “collapse of critical natural systems”, he described nature as “our most precious and irreplaceable asset” and said those systems formed “nature’s own economy”, underpinning both prosperity and national security. That passage also received a standing ovation.

The speech closed with an appeal against isolationism. “From the bitter divisions of 250 years ago, we forged a friendship that has grown into one of the most consequential alliances in human history,” he said. “I pray with all my heart that our alliance will continue to defend our shared values, with our partners in Europe and the Commonwealth, and across the world, and that we ignore the clarion calls to become ever more inward-looking.”

His final lines appeared to strike a more pointed note. “America’s words carry weight and meaning, as they have since independence,” he said. “The actions of this great nation matter even more.” The remark prompted audible murmurs in parts of the chamber before the speech ended in loud applause.

The King had opened with lighter touches, joking about the American revolution and reassuring lawmakers he was not mounting “some cunning rearguard action” on behalf of King George III. He also drew laughter with a reference to the Westminster tradition of taking an MP “hostage” during ceremonial addresses to Parliament.

Earlier on Tuesday, Charles and Queen Camilla were welcomed at the White House by President Donald Trump and First Lady Melania Trump in a ceremony marked by military pageantry and a formal arrival on the South Lawn. The King later met Trump in the Oval Office before travelling to Capitol Hill. A state dinner at the White House is due later in the evening.

The visit comes at a delicate moment in UK-US relations, with Sir Keir Starmer’s government seeking to steady ties with Trump’s White House amid strains over the war in Iran, tariff threats and wider disagreements over Europe and security burden-sharing. Against that backdrop, the King’s speech served as a high-profile display of British soft power, reinforcing London’s argument that the alliance remains strategically essential despite political tensions.

Congressional leaders had invited Charles to speak as part of events marking the 250th anniversary of American independence. In his address, the King embraced that symbolism, saying Britain and the United States had not always agreed “at least in the first instance”, but had repeatedly found ways to come together.

“Now we come together not just for the benefit of our people,” he said, “but all people.”