Charles' remarks come as the transatlantic alliance has been strained by President Donald Trump’s war against Iran.

WASHINGTON — King Charles III plans to reaffirm the United Kingdom and the United States’ long “special relationship” and common values of democracy and freedom in a rare speech to Congress on Tuesday.

The king will express “the highest regard and friendship of the British people to the people of the United States in this landmark anniversary year,” 250 years since America declared its independence from Britain, according to a Buckingham Palace official who previewed the address.

Charles' remarks come as the transatlantic alliance has been strained by President Donald Trump’s war against Iran. Trump has lashed out at British Prime Minister Keir Starmer for not joining the U.S. in the initial attack on Iran, disparaging him as “no Winston Churchill.” The president has a closer personal relationship with the king and Queen Camilla, who hosted Trump at a state dinner at Windsor Castle in September.

Despite the challenging times, the king will say in his roughly 20-minute speech on Tuesday that the U.S. and U.K. can defend and reaffirm their longstanding shared democratic values to create security and prosperity for themselves and the world, the official said.

The story of the U.S. and U.K. over the last 250 years is one of “reconciliation and renewal” and “one of the greatest alliances in human history,” Charles will say.

At the outset Tuesday, the king will refer to the assassination attempt at the White House Correspondents’ Association dinner on Saturday night. Charles and Camilla called the Trumps to express their concern and sympathies after the shooting, a source familiar with the matter told NBC News earlier this week.

Before his speech, the king will meet with House Speaker Mike Johnson, R-La., who in January made history as the first American speaker to address the British Parliament. Charles will then meet with the four top congressional leaders.

The king will deliver his remarks from the dais of the House of Representatives, where President Franklin D. Roosevelt addressed Congress in a speech the day after the Pearl Harbor attack as America entered World War II. Weeks later, Winston Churchill, would also address Congress to express the unity of the U.S. and United Kingdom in the war effort, but from the Senate chamber.

The only other British monarch to address a joint session of Congress was the king’s mother, Queen Elizabeth II, who came to the Capitol on May 16, 1991.

Her visit came a couple months after the end of the Gulf War, where the U.K. had joined the U.S.-led coalition that fought to liberate Kuwait after the Iraq invasion.

“Some people believe that power grows from the barrel of the gun. So it can, but history shows that it never grows well nor for very long,” Elizabeth said in her short address in the House chamber. “Force in the end, is sterile. We have gone a better way; our societies rest on mutual agreement, on contract and on consensus.”

The king’s grandparents, King George VI and Queen Elizabeth, made history in 1939 as the first reigning British monarchs to set foot in the United States. That trip included a visit to the Capitol but not a speech.

By Scott Wong and Jean-Nicholas Fievet