Biden met King Charles before heading to a NATO leaders summit in Lithuania

President Biden met King Charles at Windsor Castle on Monday to talk about a priority they both share: addressing climate change.

Biden and the king spoke with a group of philanthropists and business leaders from both their countries about how to finance climate aid for developing countries most affected by climate change. It included the CEOs of Bank of America, BlackRock, HSBC, Lloyd's of London and Standard Chartered.

Biden also met British Prime Minister Rishi Sunak, their sixth meeting since Sunak came into office last October. The two leaders discussed, among other things, the war in Ukraine.

This stop in London is part of a five-day trip that is largely focused on Russia's war in Ukraine. Biden and other NATO leaders will meet in Vilnius, Lithuania starting on Tuesday.

Biden has focused his foreign policy on rebuilding ties with allies — ties that he has said were severed during the Trump administration.

Sweden's membership in NATO is a key unresolved issue

Biden often points out that Russia's invasion of Ukraine prompted two historically neutral countries to apply to join the alliance: Finland and Sweden.

But Sweden's membership has been blocked by Turkey, and Hungary also has not signed off. There had been expectations that the issue could be resolved ahead of the summit, but it doesn't look likely.

Turkish President Recep Tayyip Erdogan has said Sweden isn't doing enough to clamp down on groups that it views as terrorists. But experts say there are other issues at play, too.

"President Erdogan figured he could use this as a leverage point," said Sinan Ceddi, a fellow with the Foundation for Defense of Democracies. "This has less to do with Sweden than it is to do with what he can get out of the United States, specifically which is weapons sales."

Fighter jets could become a bargaining chip

Erdogan has long wanted the United States to approve the sale of F-16 fighter jets to Turkey. The Biden administration has made it clear it supports the sale.

But the deal would need to be approved by Congress, where some lawmakers have expressed opposition, including Sen. Bob Menendez, D-N.J. the chair of the Foreign Relations Committee.

There's a growing sense in Washington that the F-16s could be a bargaining chip to get Sweden's application across the finish line.

In an interview that aired on Sunday, Biden told CNN's Fareed Zakaria GPS that he is looking for a deal to find a way to help both Turkey and Greece with aircraft.

"What I'm trying to, quite frankly, put together is a little bit of a consortium here where we ... strengthen NATO in terms of the military capacity of both Greece as well as Turkey, and allow Sweden to come in," Biden said. "It's in play. It's not done."

Biden and Erdogan — who don't have a particularly warm relationship — spoke on the phone as Biden flew across the Atlantic, and agreed to talk more during the summit.

Biden will end his trip in Helsinki, Finland to meet with Nordic leaders.

Ukraine also wants to join NATO, but the path ahead is unclear

The question of NATO expansion is not limited to Sweden. Ukraine is not a member of the military alliance, though it would like to be, and its future relationship with the alliance will be a key topic of conversation at the NATO summit in Lithuania.

Biden told CNN that bringing Ukraine into NATO during Russia's war would mean that NATO countries would be at war with Ukraine, and said that he doesn't think Ukraine is yet "ready" for membership in the alliance.

"There's other qualifications that need to be met, including democratization," Biden said.

In the meantime, even without joining NATO, Biden said the United States would be prepared to provide security aid so that Ukraine can defend itself if there should be a ceasefire with Russia, similar to what is provided to Israel.

Biden's national security adviser Jake Sullivan told reporters that would include military aid, intelligence sharing, cyber support — resources that would need to be approved by Congress.

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