Biden flies to London on Sunday on his way to a NATO summit in Lithuania. He is scheduled to hold talks with Sunak at 10 Downing St. on Monday before heading to Windsor Castle to meet Charles for the first time since the funeral of Queen Elizabeth II in September.

Though this is not a full state visit with carriage procession and palace banquet, the royal imprimatur and backdrop of the 1,000-year-old castle help underscore the importance of the trans-Atlantic “special relationship” — tested by Brexit but reinforced by unity over Russia’s invasion of Ukraine.

Sunak and the president will hold Ukraine-focused talks before both attend this week’s NATO meeting in Vilnius, which will discuss how far the military alliance should open the door to Ukraine. NATO leaders said in 2008 that Ukraine would eventually become a member, but have not set out a road map, despite impassioned entreaties from President Volodymyr Zelenskyy.

“That is an area where the U.S. is a little bit more hesitant than a lot of other NATO allies,” said Julie Norman, co-director of the Centre on U.S. Politics at University College London. “There might be some closed-door discussions about where the U.K. is on that before going into the whole (NATO) meeting.”

The U.S. and the U.K. are among the strongest Western supporters of Kyiv. Norman said that “if anything, the U.K. has taken a bit of a lead on some of the military commitments,” nudging the Biden administration to go further on issues including tanks and an international effort to give Ukraine F-16 fighter jets.

“I think in some ways that’s worked to Biden’s advantage as he’s gotten increased resistance at home from some wings of the Republican Party about not over-giving aid to Ukraine,” she said. “The fact that the U.K. is pushing and leading on this gives Biden a bit of a nudge and a bit of a strong ally support in moving ahead.”

Biden faces unease from allies including Britain about his decision to give Ukraine cluster bombs, which are banned under a convention signed by more than 120 countries, including the U.K. Sunak said Saturday that Britain “discourages their use.”

Determined to show unity among Ukraine’s allies, the U.K. has refrained from complaining about failing to secure support from Washington for Defense Secretary Ben Wallace to become the next head of NATO. Instead, the term of current Secretary General Jens Stoltenberg has been extended by a year.

The transatlantic relationship has been strained in recent years by Britain’s exit from the European Union, an act Biden has made clear he thinks harmed the U.K.

The president, who proudly celebrates his Irish roots, was especially concerned about the impact of Brexit on Northern Ireland’s peace process. Washington was relieved when Britain and the EU struck a deal in February to settle a thorny dispute over trade rules for Northern Ireland, the only part of the U.K. that shares a border with an EU member.

Biden briefly visited Belfast in April to mark 25 years since Northern Ireland’s Good Friday peace agreement, before spending several days visiting ancestral hometowns in the Republic of Ireland. He irked some in the U.K. by saying later that he’d visited Northern Ireland to make sure “the Brits didn’t screw around.”

While some U.K. Conservatives are sensitive about perceived slights from the Democrat president, this is Biden’s sixth meeting with Sunak, who has been in office since October. The British leader visited Washington last month, coming away with an “Atlantic Declaration” promising closer economic cooperation in areas including artificial intelligence, clean energy and critical minerals.

That was some consolation for Britain’s failure to get a free trade deal with the U.S., a now-buried dream of Brexit supporters .

For Sunak, standing alongside the American president provides a brief respite from his mounting domestic troubles over a stuttering economy and a fractious party. Like Biden, he faces electoral judgment next year, with a deadline of late 2024 to call a national election. U.K. inflation of 8.7% in the year to May — double the U.S. rate – is keeping millions of people in a cost-of-living squeeze, and the governing Conservatives lag behind Labour in opinion polls.

It’s also a profile-raising moment for 74-year-old Charles. Biden attended the late queen’s funeral in September, but did not come to Charles’ coronation in May, sending first lady Jill Biden instead.

Charles doesn’t have the star power of his mother, who met 13 American presidents and made more than 100 state visits during her 70 years on the throne. But he has built up a reputation as an environmental campaigner, fighting to protect wildlife and combat climate change long before it became popular.

“Charles is an important figure in the world of green and climate energy,” said George Gross, a royal historian at King’s College London. “He’s a very familiar face. So I think that there’s no disadvantage (for) a U.S. president being photographed next to him.”

Gross said the British monarch has no real political power but “a tremendous amount of soft power” because so many world leaders want to experience the royal mystique.

“There are things the U.K. government can push through Charles, if they want to, that can be said and talked about in a way that can’t be done by the prime minister in quite the same way,” he said.