Washington DC - Britain's King Charles III urged the US 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.
"The challenges we face are too great for any one nation to bear alone," he said in a speech at the US Congress, calling for partners to defend shared values.
"Whatever our differences, whatever disagreements we may have, we stand united in our commitment to uphold democracy," the monarch told lawmakers.
Addressing Congress during celebrations marking 250 years since American independence from Britain, Charles stressed the historic ties between the two nations, saying their partnership was "born out of dispute, but no less strong for it."
"I pray with all my heart that our alliance will continue to defend our shared values... and that we ignore the clarion calls to become ever more inward-looking," he added.
He said "unyielding resolve is needed for the defense of Ukraine and her most courageous people – in order to secure a truly just and lasting peace."
President Donald Trump – hosting the royal couple for four days – earlier hailed Britain as America's closest ally as he welcomed Charles and Queen Camilla to the White House on a state visit overshadowed by tensions over the Iran conflict.
Speaking after a pomp-filled welcome on the South Lawn featuring a 21-gun salute, Trump's tone was a world away from his recent jibes at British Prime Minister Keir Starmer for failing to join the US war against Tehran.
Charles emphasizes historic US-UK cooperation
The king's address to Congress – the first by a British monarch since Queen Elizabeth II in 1991 – is the centerpiece of the state visit.
In his speech, Charles highlighted shared democratic traditions dating back centuries, describing Congress as a "citadel of democracy" and stressing the common roots of British and American law.
Charles said Britain's foundational legal document, Magna Carta, had been cited in an estimated 160-plus Supreme Court cases since 1789 – noting, to applause from the opposition Democrats, the principle that "executive power is subject to checks and balances."
He also pointed to deep defense and economic links, from joint military programs to cooperation in emerging technologies, while warning that security challenges are growing more acute.
"Our defense, intelligence, and security ties are hardwired together," he said.
The royal couple will attend a state dinner later on Tuesday, capping a visit that has mixed ceremony with sensitive diplomacy.
Security has been tight for the trip, which comes just days after an alleged assassination attempt against Trump at a Washington press gala. Counter-sniper teams could be seen on the roof of the White House.
Melania Trump told reporters she was "very well" when asked how she was doing after the shooting.
"Let me say with unshakeable resolve: such acts of violence will never succeed," King Charles told lawmakers.