"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," Charles told US lawmakers on Tuesday during a rare speech to a joint meeting of the Senate and House of Representatives, and after a prolonged standing ovation at his entrance with Queen Camilla.
In a speech that Buckingham Palace said would not be political, Charles also made references to President Donald Trump's criticism of NATO, highlighted the importance of continued US help for Ukraine in its war with Russia, the dangers of isolationism, and even a call for the safeguarding of nature, an important issue to the King for most of his life.
Trump has been highly critical of NATO and European allies, most recently over their reluctance to provide military help in the US-Israeli war against Iran.
Trump has also been ambivalent about prolonged US financial and military aid to Ukraine.
Charles referred to the September 11, 2001, attacks, saying, "we answered the call together, as our people have done so for more than a century, shoulder to shoulder through two world wars, the Cold War, Afghanistan and moments that have defined our shared security today.
"Mr Speaker, that same unyielding resolve is needed for the defence of Ukraine, and her most courageous people."
Later, at a state banquet at the White House, Trump said Charles did not want Iran to have a nuclear weapon, a statement the King did not comment on.
Trump said "we have militarily defeated that particular opponent", and "Charles agrees with me even more than I do - we're never going to let that opponent have a nuclear weapon".
In his own comments after Trump spoke, Charles did not speak about Iran or the Iran war.
Earlier, in his comments to Congress, in what appeared to be a reference to Trump's "America First" agenda, Charles said, "I pray with all my heart that our lands 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."
The King was only the second British sovereign to address the US Congress.
His mother, Queen Elizabeth II, spoke to both houses in 1991.
Later, Charles met US tech leaders, including Amazon founder Jeff Bezos, Apple CEO Tim Cook and Nvidia CEO Jensen Huang, discussing challenges for early-stage start-ups as the UK touts itself as a top destination for technology firms.
Charles' speech came on the second day of a four-day visit marking 250 years of American independence, and during a tense time in relations between the two countries after Trump has repeatedly criticised UK Prime Minister Keir Starmer for what Trump says is his lack of help in prosecuting the Iran war.
As a British monarch, Charles' speech was written on the advice of the UK government.
During a reception at the White House, Trump stressed the friendship that has evolved between Britons and Americans since their days as adversaries during the War of Independence.
After escorting the King and Queen to their limousine for departure from the White House, Trump told reporters: "It was a really good meeting. He's a fantastic person. They're incredible people and it's a real honour."
One topic that Charles has sought to avoid is the scandal surrounding convicted sex offender Jeffrey Epstein, which has roiled politics in both the US and UK, and ensnared officials including the King's brother, Andrew.
with AP
