King Charles rolls out the red carpet to woo Trump

King Charles III, Prince William and more than 1,500 soldiers will welcome President Trump to Windsor Castle on Wednesday in an effort to butter up the U.S. president and cement a diplomatic relationship with the most Anglophile U.S. leader in decades.

Over two days, the grounds around the thousand-year-old castle will be turned into a regal theme park, where Trump and the first lady will travel in a horse-drawn carriage to the sound of military bands, witness firing cannons and a flyover, and be the guests of honor at a lavish banquet. In doing so, Trump will be the first U.S. president to have had two such state visits organized in his honor, after a previous trip to Buckingham Palace in 2019.

The rollout of this royal red carpet is already having its desired effect. Trump gushed about his coming trip in remarks to reporters on Sunday evening, pointing out that he was making royal history. “They’ve never done it before, twice honored," he said. “It’s going to be very exciting."

For the British government, which decides who gets such state visits, much is at stake.

The U.K. government has so far leveraged Trump’s soft spot for Britain to try to navigate his global trade war, securing an early tariffs deal that is so far lower than the levies outlined for most other countries. Prime Minister Keir Starmer is hoping that a renewed dose of pomp and ceremony will soften Trump up for further hardheaded discussions around steel tariffs and efforts to bolster U.S. support for European security when the two men hold a bilateral meeting on Thursday.

“Starmer is using the U.K.’s monarchy as a diplomatic arsenal very effectively, and that may have helped him avoid the worst social-media tirades or harsher tariffs in the trade wars," said Lewis Lukens, a former career U.S. ambassador who served as a senior U.S. Embassy official in London during Trump’s first term. “Trump does have a soft spot for the U.K. and for the royal family."

The visit by Trump will be timed with announcements of greater U.S. and U.K. cooperation in technology, financial services, artificial intelligence and nuclear energy. However, British officials conceded nothing is assured with Trump. Potential flashpoints include the U.K. government’s decision to potentially recognize Palestinian statehood in coming weeks and a choice last week to remove its Washington ambassador over his past friendship with disgraced financier Jeffrey Epstein.

U.S. officials said the state visit wouldn’t only highlight the “special relationship" between the two countries, but also celebrate a slightly more delicate issue for the royal family: the coming 250th anniversary of America’s founding, when the country ditched the British monarchy for its independence.

Over the past century the British monarchy has completed a foreign-policy transition from rulers of a vast empire to celebrity diplomats. Regal soft power is becoming an increasingly important crutch for the British government as the country’s economic and military might in the world diminishes. The “special relationship" between the U.K. and the U.S., in particular, has become ever more lopsided—so much so that the prospect of an overnight stay with a real king is vital in keeping the leader of the free world sweet.

Trump has often spoken of his regard for the late Queen Elizabeth II, whom his Scottish mother much admired. He also has a fondness for Charles, whom he made an honorary member of his Mar-a-Lago club back in the 1990s.

Shortly after being re-elected in November, Trump was shown in Tucker Carlson’s docuseries “Art of The Surge" leafing through a photo album of his state visit to meet Queen Elizabeth when he was first president. “I mean who has images like this," Trump said. “And these were relationships, too…Look, it’s Charles. So beautiful," he said about a photo.

Starmer’s government wasted no time wooing Trump. A good relationship wasn’t a sure thing given many voices in Trump’s MAGA movement, including Elon Musk, have been sharply critical of the U.K. over issues such as immigration and free speech, and given Starmer leads a left-of-center Labour government. When Starmer visited with Trump in February, one of his first actions was to hand over a signed note from Charles inviting him to a state visit to which Trump responded by saying, “Oh wow."

Seasoned British royal watchers weren’t impressed by the stunt, as personal invitations from the king aren’t normally waved in front of the masses. “It was like seeing someone with their trousers down," Robert Hardman, a royal biographer, quoted one royal official in his book “Charles III". “These things should be done with dignity."

The novelty factor of holding the visit at Windsor, as Buckingham Palace is currently being refurbished, is another lure for Trump. “Windsor is supposed to be amazing," he told reporters ahead of his trip. “They’ve never had Windsor for this." Trump and his wife will visit Queen Elizabeth’s tomb located in the castle chapel to lay a wreath. The couple will also meet with Prince William and Catherine, Princess of Wales.

This reliance on the monarch by the British government isn’t without a certain awkwardness for King Charles—whose interests at times sit diametrically opposed to the U.S. president’s. Charles, for instance, is a great defender of environmental causes.

“Trump is where we are seeing King Charles being used most effectively, regardless of whether he wants to be used that way," said Evie Aspinall, director at the British Foreign Policy Group, a think tank. “There will be situations where Charles is quite uncomfortable."

Charles is also king of Canada, a country that Trump repeatedly mused about annexing early in his administration in comments that infuriated Ottawa. Charles is a keen supporter of Ukraine, a country whose sovereignty risks being undermined if Trump doesn’t give it lasting security guarantees as part of any deal to end its war with Russia. Finally, polls show Trump isn’t popular with the British people, which makes King Charles lavishing attention on the U.S. president look out of step with his subjects’ views. Any crowds will be kept well away from Windsor grounds.

King Charles can’t publicly speak without government approval, so any personal feelings won’t be on display, officials say. However the monarch hasn’t been shy in veiled displays dissenting with the U.S. president. Shortly after Trump berated Ukrainian President Volodymyr Zelensky in the White House earlier this year, Charles was photographed inviting Zelensky for tea.

Earlier this year, King Charles visited Ottawa to open Canada’s parliament in a pointed symbol of support for the country. Trump’s public statements about Canada seemed to dial down noticeably after, Aspinall said.

The big question now, say royal watchers, is whether Trump will return the favor and invite King Charles on a state visit to Washington.

Write to Max Colchester at Max.Colchester@wsj.com, Robbie Gramer at robbie.gramer@wsj.com and Tarini Parti at tarini.parti@wsj.com