The official reason for the royal trip was to mark the 250th anniversary of the US winning its independence from British colonial rule, cueing multiple wry jokes from Charles in speeches to Washington DC's elite about being on the losing side of the American Revolutionary War.
But it was also designed to mend what Charles called in Tuesday's state dinner with Trump an "unbreakable bond" and "indispensable alliance" between the two countries, lately strained by the United Kingdom, alongside other European allies, declining to join the two-month-old US-Israeli war against Iran.
It seemed to work.
As enraged as he has been by UK Prime Minister Keir Starmer, Trump told reporters at some length how fond he was of his "great friend" Charles the day after their dinner: "When you like the king of a country so much, it probably helps your relationship with the prime minister."
Posing for photographs on a red carpet outside the White House's South Portico on Thursday morning, Trump - frequently denounced by domestic political opponents as a would-be king - pointed to the monarch and said: "He's the greatest king, in my book."
The two men, joined by Camilla and US first lady Melania Trump, went inside, came back out five minutes later, and the royals got in their car to tour several sites in Virginia.
"Great people," Trump said toward the departing motorcade.
"We need more people like that in our country."
Trump said on Thursday he was removing certain US tariffs on Scotch whisky after the royal visit.
"The King and Queen got me to do something that nobody else was able to do, without hardly even asking!" Trump posted on social media.
Trump said that people had wanted this change, especially with regard to the wooden barrels in which the spirits of Scotch and bourbon can be aged.
During the royal trip, Charles has drawn smiles from lawmakers in the US Congress to young Harlem school children at an urban farm in New York City.
Among the biggest smiles of all came from Trump himself, as Charles revealed a gift for the president at Tuesday's White House reception: the original bell that hung from the conning tower of a Royal Navy submarine launched from a UK shipyard in 1944 and named HMS Trump.
For his final day, Charles is expected to lay a wreath at Arlington National Cemetery, across the Potomac River in Virginia, a sacred site for many where tens of thousands of the country's war dead are buried.
On Wednesday the King and Queen commemorated victims of the September 11, 2001 attack on New York City, laying a floral bouquet at the memorial where the World Trade Center's twin towers once stood.
The royal couple are also expected to attend a small-town block party in Virginia to join in what the UK embassy called the "North American tradition" of "a 'potluck' meal".
Later in the day, the royal couple will fly to Bermuda for Charles' first visit as sovereign to the British territory.
with AP
