Leaders of the 31 countries that comprise the North Atlantic Treaty Organization will gather in Vilnius, Lithuania, for a two-day summit starting Tuesday, as Russia’s war in Ukraine nears its 18-month mark.

The meeting comes at a crucial moment in the conflict. NATO is seeking to bolster Kyiv with fresh munitions — including U.S.-provided cluster bombs — for its counteroffensive, and gauge the impact of June’s aborted mutiny by Wagner Group head Yevgeniy Prigozhin on Russia’s leadership and operations.

But the alliance is also grappling with internal squabbling over Turkey’s reluctance to approve Sweden’s membership and whether to pave a path for Ukraine’s eventual accession.

Nevertheless, the assembled leaders — including U.S. President Biden, still the alliance’s most powerful figure — are eager to use this week’s meeting to signal that the conflict in Ukraine has only strengthened NATO. Biden has also planned a high-profile climate meeting with Britain’s King Charles III and a meeting with Nordic nations hosted by Finland, the alliance’s newest member.

Here’s what to watch for as world leaders descend on Vilnius:

Sweden membership

Turkish President Recep Tayyip Erdogan is scheduled to meet Swedish Prime Minister Ulf Kristersson on Monday ahead of the summit, as the Nordic nation makes a last-ditch effort to convince Ankara that it should be allowed to join NATO.

Turkey has said its opposition stems from concerns Sweden isn’t doing enough to clamp down on Kurdish separatist groups that Ankara views as terrorist organizations.

Erdogan has also signaled his eagerness for a meeting with Biden — U.S. National Security Adviser Jake Sullivan said he expects the pair to talk at the summit — and to purchase American F-16 fighter jets for his military.

Biden acknowledged that “Turkey is looking for modernization of F-16 aircraft” and suggested it could be part of a U.S. move to also strengthen neighboring Greece militarily. “It’s in play,” he said in a July 7 interview for CNN’s Fareed Zakaria GPS. “It’s not done.”

He renewed his support for Sweden to join NATO, adding “I’m optimistic.”

While it’s too late for Turkey and fellow holdout Hungary to ratify Sweden’s membership before the summit, allies are holding out hope that the two sides can announce they have overcome the impasse.

NATO Secretary General Jens Stoltenberg sounded optimistic on Thursday, saying Sweden’s membership is within reach and that it was possible to have a “positive decision” at the summit. A day later, Erdogan said Turkey can’t trust a country where “terrorists roam free in its streets.”

Ukraine aid

Ukraine will be a key topic, with President Volodymyr Zelenskyy planning to participate in the summit.

The 31 NATO countries are expected to offer Kyiv a promise of long-term support, which aims to deepen ties without immediately making it a member, given that the bloc’s Article 5 security guarantees could draw allies into Russia’s war against Ukraine. The United States announced an $800 million package Friday that includes controversial cluster munitions, which some NATO allies have outlawed over humanitarian concerns related to unexploded ordinance.

Biden in a Friday interview with CNN said that Ukraine isn’t “ready for membership in NATO,” for reasons including Russia’s ongoing assault on the country and NATO’s Article 5 provisions.

Zelenskyy has called for the summit to send clear signals in support of his country’s membership, urging allies to provide a more concrete perspective beyond a 15-year-old statement that Ukraine will eventually join.

The allies are grappling with how to address the question in the summit statement, with some eastern NATO members pushing for a more concrete path. Countries like the United States and Germany have wanted to focus instead on immediate assistance. One option could entail declaring Ukraine doesn’t need a Membership Action Plan — a way to fast-track the country’s membership when allies eventually decide to invite it to join.

The Vilnius package will upgrade the formal status of NATO’s relationship with Ukraine by establishing a new NATO-Ukraine Council, allowing the country to directly take part in broader discussions about the alliance’s security and hold consultations with allies about its security concerns.

NATO leaders are also expected to agree to a €500 million a year fund in nonlethal aid to help Ukraine modernize its military. On the sidelines of the summit, some allies are expected to pledge bilateral security assurances to Ukraine, committing to ensure its armed forces are well-equipped and well-trained in an effort to deter Russia from re-invading after the war ends.

Defense spending

NATO leaders are due to sign off on a new defense spending pledge, making an enduring commitment to spend “at least” 2 percent of gross domestic product on defense. The agreement extends the alliance’s previous aspirational goal of targeting 2 percent and underscores vows to spend more following the Ukraine invasion.

But many nations — including Luxembourg, Canada and Italy — are still struggling to comply with the old guideline. Only 11 of the 31 allies are expected to meet the 2 percent goal this year, according to estimates published by NATO on Friday.

The alliance is also expected to sign off on three regional defense plans for the first time since the end of the Cold War, which spell out in detail how countries will defend territory if it comes under attack by Russia or terror groups.

Leaders are also set to endorse a defense industry action plan, aimed at boosting defense production as Ukraine burns through artillery ammunition more quickly than allies can produce it.

Secretary General

One of the biggest questions facing the assembled leaders in Vilnius is who will lead their group into the future, especially as the conflict in Ukraine threatens to drag on.

Stoltenberg agreed last week to serve another year — his fourth extension in the top job — despite previously stating publicly that he didn’t seek to prolong his post.

But neither top candidate to replace him — Danish Prime Minister Mette Frederiksen or British defense chief Ben Wallace — appears able to corral enough of a consensus to secure the job, and the United States ultimately refrained from publicly endorsing a candidate. Biden’s top priority was maintaining unity within the alliance, according to a person familiar with the matter.

Stoltenberg’s return has fanned speculation that European Commission President Ursula von der Leyen, a former German defense minister, could be in line for the job after her term expires next year.

Special relationship

Biden’s visit to the U.K., his second in three months, is largely seen as a make up trip after he declined to attend Charles III’s coronation in May.

Although Biden plans to visit 10 Downing Street to meet Prime Minister Rishi Sunak before traveling to Windsor Castle, he’s expected to be on the ground for less than a day. During his royal audience, Biden and the British monarch are expected to unveil a dual effort toward a mutual passion — recruiting private companies to help fund projects that can reduce climate change.

The British will be looking for Biden to firm up his commitment to a series of minor trade, military and technology agreements Sunak touted as the “Atlantic Declaration” while visiting the White House last month.

Biden could also put Sunak on the spot by pressing for a solution to the stalemate over implementing power-sharing provisions of the Good Friday Agreement. Expect a firm pledge of military support for Ukraine, with the UK second only to the United States in providing arms and aid to the Ukrainians.

Brendan Scott contributed to this report.