King Charles III urged the United States on Tuesday to stand firm with its Western allies, as strains over Iran and Ukraine risk undermining transatlantic ties facing their most serious test in years.

In a rare address to Congress, the monarch skirted around tensions between President Donald Trump and British Prime Minister Keir Starmer, insisting the partnership between the two countries was “born out of dispute, but no less strong for it.”

The speech was warmly received, even as he ranged over subjects from climate change and the need for restraints on presidential power to the importance of NATO and the defense of Ukraine -- sensitive issues for those on the Republican side of the aisle.

“The challenges we face are too great for any one nation to bear alone,” Charles said, warning that global challenges require closer cooperation between the United States and its traditional partners.

Addressing lawmakers during celebrations marking 250 years since American independence, Charles stressed that “unyielding resolve” was needed to secure a “just and lasting peace” in Ukraine, which has been fighting a full-scale invasion by Russia since 2022.

Trump earlier hailed Britain as America’s closest ally as he welcomed Charles and Queen Camilla to the White House on a four-day state visit.

Speaking after a pomp-filled welcome on the South Lawn featuring a 21-gun salute, Trump struck a markedly warmer tone than in recent criticism of Starmer.

- ‘No closer friends’ -

“In the centuries since we won our independence, Americans have had no closer friends than the British,” Trump said, hailing the “special relationship” and the two countries’ military partnership.

Cannons rang out while a military band played the British and US national anthems, before Charles shook hands with top Trump administration officials and inspected troops on the lawn with the president. 

Afterward, four US jets roared over the White House in a noisy flypast as Trump, Charles, Camilla and First Lady Melania Trump watched.

The visit comes at a delicate moment, with Trump criticizing Britain over its stance on Iran and other policies.

Despite the tensions, the US president struck a jovial tone, joking about his Scottish-born mother having “a crush on Charles” ahead talks in the Oval Office.

“He’s a fantastic person. They’re incredible people and it’s a real honor,” Trump told reporters after the meeting, which took place behind closed doors.

- ‘Checks and balances’ -

The king’s address to Congress -- the first by a British monarch since Queen Elizabeth II in 1991 -- was the centrepiece of the visit.

Charles pointed to shared democratic traditions, describing Congress as a “citadel of democracy” and stressing the common legal roots of the two nations.

He noted that Magna Carta had been cited in more than 160 US Supreme Court cases, highlighting  -- to rapt applause from the opposition Democrats -- the principle that executive power is subject to checks and balances.

The monarch also emphasized deep defense and economic ties, saying cooperation in security, technology and trade remains central to the alliance.

“Our defense, intelligence and security ties are hardwired together,” he said.

Security has been tight during the visit following an alleged assassination attempt against Trump at a weekend Washington media gala.

“Such acts of violence will never succeed,” Charles said.

The royal couple will attend a state dinner later on Tuesday, capping a visit that has mixed ceremony with sensitive diplomacy.

- Agencies