Biden is heading to Europe. A king and a war are on his agenda

President Joe Biden leaves on Sunday for Europe, where he will spend four days in three nations tending to alliances that have been tested by Russia's invasion of Ukraine.

The first stop is London, where Biden will meet with King Charles III for the first time since his ascension to the throne. After that is the centerpiece of the trip, the NATO summit in Vilnius, Lithuania. Alliance leaders will debate the war and revise plans for dealing with Russian aggression.

The final stop is in Helsinki, where Biden is expected to celebrate the expanding alliance, with Finland as the newest member of NATO.

His national security adviser, Jake Sullivan, said the trip would “showcase the president's leadership on the world stage.”

A look at Biden's agenda and the issues he will face:

London

Biden arrives in London on Sunday night is expected to have a full schedule of meetings Monday.

“There’s always a lot to talk about with the U.K.,” said Max Bergmann, a former State Department official who leads the Europe Program at the Center for Strategic and International Studies.

Biden will hold talks with Prime Minister Rishi Sunak at 10 Downing St. Sunak is facing an election by the end of next year. His Conservative Party is lagging badly behind the opposition in opinion polls.

Despite Sunak’s shaky political standing, he has fostered close ties with Biden and it will be their sixth meeting since Sunak took office last October.

Bergmann said Sunak's tenure has been a nice change of pace after “there were some concerns about Boris Johnson,” one of Sunak’s predecessors, “being a loose cannon.”

Biden will visit the king at Windsor Castle, a royal residence outside London. Biden did not attend Charles' coronation — first lady Jill Biden went in his place — so this will be their first encounter since then.

They're expected to discuss climate change, an issue that has been a focus for both leaders, and how to finance initiatives to address the problem.

Vilnius

Biden will spend two days in the capital of Lithuania, which is hosting the annual NATO summit. He will participate in meetings with leaders and deliver a speech from Vilnius University.

The alliance has been reinvigorated by the war in Ukraine, and members have been pouring military hardware into the country to help repel Russia's invasion.

Biden on Friday defended what he said was a “difficult decision” to provide cluster munitions to Ukraine, a move his administration said was key to the fight and buttressed by Ukraine’s promise to use the controversial bombs carefully. Biden is likely to face questions from allies on why the U.S. would send a weapon into Ukraine that more than two-thirds of NATO members have banned because it has a track record for causing many civilian casualties.

For Secretary-General Jens Stoltenberg, the summit “will send a clear message: NATO stands united, and Russia’s aggression will not pay.”

But NATO has also struggled to bridge divides over important issues. Finland was welcomed into the alliance this year, but Sweden's membership has been blocked by Turkey and Hungary.

There are also disagreements over how quickly to extend an invitation to Ukraine to join NATO.

Countries on NATO's eastern flank want to move quickly, viewing it as a way to deter Russian aggression. The U.S. and others advocate a more cautious approach.

One issue has already been settled, at least for the time being. Stoltenberg's term has been extended for a year because members could not agree on a new leader.

Sen. Thom Tillis, who will attend the summit, likened the alliance to a gathering of dozens of family members who bicker and clash but nonetheless remain united.

“At the end of the day, you know you’re family," said Tillis, R-N.C.

Tillis is leading a bipartisan delegation along with Sen. Jeanne Shaheen, D-N.H., who said NATO is more powerful than before.

"It’s the strongest military alliance in our history, and I think it only has gotten stronger as a result of U.S. leadership, as the result of Stoltenberg’s leadership and as the result of the threat from Vladimir Putin to all of the NATO allies and other countries in Europe and around the world and to the international order,” she said.

Helsinki

After two nights in Vilnius, Biden visits Helsinki. The stop is a bit of a victory lap, but could also be a reminder of unfinished business.

The Nordic country in April became the 31st member of NATO, ending its history of nonalignment and demonstrating how Russia's invasion of Ukraine has backfired in Europe.

Finland was supposed to join alongside its neighbor Sweden, whose admission has stalled. NATO requires unanimous consent of all its members to expand, and the U.S. has been unable to overcome objections from Turkey and Hungary.

Swedish Prime Minister Ulf Kristersson visited the White House on Wednesday and met with Biden to keep up the pressure for membership. But there is little hope that the issue will be resolved in Vilnius.

The White House is billing Biden's visit to Helsinki as a “U.S.-Nordic Leaders Summit.”

It's a much different occasion from the last time a U.S. president visited Helsinki five years ago.

During that trip, Donald Trump held a news conference with Putin and brushed off concerns about Russian meddling in the Trump's election victory.

Now Biden is heading to the city to demonstrate how his administration has held the line against Moscow and expanded Western defenses.

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Associated Press writer Lorne Cook in Brussels contributed to this report.