Inside the Biden administration's push to get Sweden into NATO and F-16s to Turkey

A full-court press by the Biden administration in the days leading up to the NATO summit capped months of behind-the-scenes diplomacy to get Turkey to move ahead with Sweden's accession to NATO.

A major part of that diplomatic effort: Getting progress on the potential sale of F-16 fighter jets to Ankara - a request that, despite claims by top US officials, had become linked to the question of Sweden's membership to NATO.

The sale of the US arms to Turkey is not a done deal, and the key opponent, Senate Foreign Relations Committee Chairman Bob Menendez, told CNN on Tuesday afternoon that "as of now" he remains opposed.

The New Jersey Democrat has long been against selling the fighter jets to Ankara, not only due to Turkey's roadblocks for Sweden's membership, but also its human rights issues and regional aggressions including its tensions with Greece.

However, there are indications that efforts to shift Menendez's position and address his concerns have had some impact.

New details reported by CNN show how months of steady diplomacy, helped along by the close relationship of two former Senate colleagues, resulted in what could become one of the biggest foreign policy accomplishments of the Biden administration.

Now all eyes are on a meeting on Wednesday between Turkish President Recep Tayyip Erdogan and Greek Prime Minister Kyriakos Mitsotakis to see if the two leaders can do enough to assuage the US lawmaker's concerns and gain the approval of the F-16 sale.

Ahead of the high-stakes meeting Wednesday, the Biden administration expects that Erdogan and Mitsotakis will commit to keeping the calm in the region and possibly strike an agreement to respect one another's airspace, after a year of unprecedented Turkish incursions into Greek airspace, a US official familiar with efforts and a regional diplomat told CNN. The hope is that such a statement could address Menendez's concerns.

After Finland and Sweden were invited to join NATO, efforts got underway to ensure their membership in the defensive alliance. Turkey put up obstacles to both countries, but eventually allowed Helsinki to move ahead. However, Ankara continued to maintain opposition to Sweden, making demands related to Kurdish terror groups, like the Kurdistan Workers' Party (PKK), and arms sales.

Behind the scenes, the F-16 sale was also operating as an implicit demand by Ankara, and US diplomats got to work on extensive engagements with both counterparts and lawmakers.

Among those engagements were multiple trips to Washington, DC, by US Ambassador to Turkey Jeff Flake.

Flake's experience as a former US senator from Arizona and his personal relationship with Menendez were key as he tried to explain his former colleague's position to Turkish officials. He simultaneously worked with his former colleague to figure out how the New Jersey Democrat could get to a "yes" on the F-16 sale, a source told CNN.

Flake, along with US Ambassador to Greece George Tsunis, met with lawmakers on the Senate Foreign Relations Committee and House Foreign Affairs Committee about the potential arms sales to the two countries, said a source familiar with the talks.

Flake also visited Capitol Hill multiple times in the spring with Erdogan's top national security advisor, Ibrahim Kalin, with the goal of introducing him to members of Congress, the source said. The US ambassador wanted to ensure that Kalin understood how important Sweden joining NATO was to a wide array of members of Congress, the source said.

Turkish officials also got that message from the bipartisan letter earlier this year, spearheaded by Democratic Sen. Jeanne Shaheen and Republican Sen. Thom Tillis, in which 28 senators said they would not back the F-16 sale to Turkey so long as Ankara continues to block Sweden from joining NATO, the source said.

The Biden administration's engagements with the Hill went beyond the visit by the top US diplomats in Greece and Turkey.

US Secretary of State Antony Blinken had been in touch with members of Congress, including Menendez, about what it would take to get to a "yes" on sending F-16s to Turkey, a senior State Department official said.

He "had conversations with members of Congress, including Sen. Menendez about this very issue in the past few weeks," State Department spokesperson Matthew Miller said Tuesday.

Meanwhile, administration officials were also engaging extensively with foreign counterparts to try to move Turkey forward on Sweden's NATO bid - a win that was achieved on Monday, the eve of the summit.

"When the NATO Summit gets underway, our alliance will not only be bigger and stronger than ever, it will be more united, more purposeful, and more energized than at any point in modern memory. And that is in no small part thanks to President Biden's personal leadership," Sullivan told reporters Tuesday morning.

Biden phoned Erodgan from Air Force One en route to London on Sunday to discuss both Stockholm's NATO membership and the F-16 sale.

"President Biden has been clear consistently that he believes that, for the alliance and for the US-Turkey bilateral relationship, moving forward with the sale makes sense," Sullivan told reporters shortly after their call, adding on Tuesday that Biden "intends move forward with that transfer in consultation with Congress."

Blinken spoke three times with the Turkish foreign minister in the five days leading up to the summit, and Sullivan spoke with his Turkish and Swedish counterparts as recently as Monday.

Blinken and Biden also spoke separately with Mitsotakis in recent weeks, as the administration prepared for this week where they hoped to pull off an intense diplomatic effort.

Biden himself signaled last week that such an agreement could be coming together, telling CNN's Fareed Zakaria in an exclusive interview, "What I'm trying to, quite frankly, put together is a little bit of a consortium here, where we're strengthening NATO in terms of military capacity of both Greece as well as Turkey, and allow Sweden to come in. But it's in play. It's not done."

Menendez told CNN on Monday that he needs to see Turkey commit to "less hostility to a fellow NATO ally" - meaning Greece - "commitment that the tranquility that has existed over the last several months continues," and "no use of US weapons against another NATO ally."

Tensions between Athens and Ankara have diffused in recent months, due in part to the catastrophic earthquake in Turkey, for which Greece provided assistance. US officials urged both sides to maintain that calm, the source familiar said.