Donald Trump and the King appeared to be in a “grateful” mood after a military ceremony and Red Arrows flypast at Windsor Castle.
The US President spoke to Charles several times during Beating Retreat on the East Lawn on Wednesday afternoon, and even patted him on the arm.
It is the first time the ceremony has been performed at an incoming state visit.
The King and Mr Trump were seated in chairs next to the Queen and the First Lady, alongside an audience including Sir Keir Starmer and his wife, Lady Starmer and British and US military personnel.
The Massed Bands of the Royal Marines, the Coldstream Guards and the Royal Air Force, with the Old Guard Fifes and Drums from the US Army Band, took part.
During the display, the King appeared to point out the Old Guard Fifes and Drums to Mr Trump as the band moved towards them from the right.
The quartet spoke with each other during the display, with Camilla seen at one point laughing and holding on to her hat amid windy conditions.
After the 30-minute ceremony, the Red Arrows went over and the King and Queen and US president and first lady all looked up.
They then applauded and Mr Trump smiled and leaned towards Charles, spoke to him and patted him on the arm.
The pair then talked with military personnel who were sitting on their left while Camilla and Mrs Trump spoke to those on the other side.
Beating Retreat is a military spectacle and ceremony that originated in the early years of organised warfare and symbolises the end of the day when camp gates were closed and ceremonial flags were lowered.
Lieutenant Colonel Simon Soskin of the Grenadier Guards, who was tasked with planning the ceremony, told the PA news agency: “We were asked to put on a spectacular performance to end the day of welcoming and so we pulled in our three bands, one for each service, invited the Americans, sprinkled in some bagpipers and it came together very well.”
Asked what the King and US president said to the gathered military personnel who watched the ceremony, Lt Col Soskin said: “They took a few moments to say hello.
“I said hello to the president on the way past, which I like because my family are American, so I’ll be telling my parents later.
“They were grateful, I think, the President was really pleased to see the junior American service personnel.”
