Washington — Saying he felt the "weight of history" on his shoulders, King Charles became the first British monarch in 35 years to address Congress on Tuesday, where he urged lawmakers to rededicate their commitment to the transatlantic U.S.-U.K. alliance and resist "clarion calls to become ever more inward-looking."

"America's words carry weight and meaning, as they have since independence. The actions of this great nation matter even more. President Lincoln understood this so well, with his reflection in the magisterial Gettysburg Address that the world may little note what we say, but will never forget what we do," the king told a packed House chamber on the second day of his state visit to the U.S. "And so, to the United States of America, on your 250th birthday, let our two countries rededicate ourselves to each other in the selfless service of our peoples and of all the peoples of the world."

This week's visit from the king and Queen Camilla comes as the relationship between President Trump and British Prime Minister Keir Starmer has been strained over the war in Iran and other issues. In his nearly 30-minute speech, Charles highlighted the centuries-long relationship between the U.S. and U.K.

"The modern relationship between our two nations and our own peoples spans not merely 250 years, but over four centuries. It is extraordinary to think that I am the nineteenth in our line of sovereigns to study, with daily attention, the affairs of America," he said. "So I come here today with the highest respect for the United States Congress, this citadel of democracy created to represent the voice of all American people to advance sacred rights and freedoms."

The king noted that the speech came "in times of great uncertainty, in times of conflict from Europe to the Middle East which pose immense challenges for the international community and whose impact is felt in communities the length and breadth of our own countries." 

Charles said that the two nations "do not always agree" but his remarks largely focused on their partnership, referring to it as "one of the most consequential alliances in human history." 

"Our two countries have always found ways to come together," he said, calling the U.S.-U.K. alliance "truly unique." "That partnership … is more important today than it has ever been." 

"The story of the United Kingdom and the United States is, at its heart, a story of reconciliation, renewal and remarkable partnership," he added. 

He said that "the challenges we face are too great for any one nation to bear alone."

"But in this unpredictable environment, our alliance cannot rest on past achievements, or assume that foundational principles simply endure," he continued. "As my prime minister said last month: 'Ours is an indispensable partnership. We must not disregard everything that has sustained us for the last eighty years. Instead, we must build on it.'"

The king briefly mentioned Saturday's White House Correspondents' Dinner shooting, where the president and some members of Congress were present, declaring that "such acts of violence will never succeed." The line was one of several that received a standing ovation from lawmakers. 

"Whatever our differences, whatever disagreements we may have, we stand united in our commitment to uphold democracy, to protect all our people from harm, and to salute the courage of those who daily risk their lives in the service of our countries," he said of Saturday's incident. 

The king referenced NATO, an alliance Mr. Trump has threatened to leave, saying "our defense, intelligence and security ties are hardwired together through relationships measured not in years, but in decades." He also mentioned several ways in which the nations have partnered on defense, including building F-35s and a submarine partnership between the U.S., U.K. and Australia. 

"We do not embark on these remarkable endeavors together out of sentiment. We do so because they build greater shared resilience for the future, so making our citizens safer for generations to come," he said. 

He said the same "unyielding resolve" that the U.S. and U.K. showed in World War II, the Cold War and after 9/11 "is needed for the defense of Ukraine and her most courageous people, in order to secure a truly just and lasting peace," a line that got a bipartisan standing ovation from those in the chamber.

It was the second time ever that a British monarch has addressed Congress. Queen Elizabeth II, Charles' mother, was the first in 1991, which Charles noted.

"Speaking in this renowned chamber of debate and deliberation, I cannot help but think of my late mother, Queen Elizabeth, who, in 1991, was also afforded this signal honor and similarly spoke under the watchful eye of the statue of freedom above us," he said.

Before addressing lawmakers, the king met with Senate Majority Leader John Thune, House Speaker Mike Johnson, Senate Minority Leader Chuck Schumer and House Minority Leader Hakeem Jeffries. 

Mr. Trump and first lady Melania Trump welcomed the king and queen to the White House earlier Tuesday with a formal arrival ceremony. Mr. Trump referenced the "special relationship" between the U.S. and the United Kingdom, saying "we hope it will always remain that way."