President Biden, left, walks with Ukrainian President Volodymyr Zelensky ahead of a working session on Ukraine during the G7 Summit in Hiroshima, Japan on Sunday, May 21. Susan Walsh/Pool via Associated Press

WASHINGTON — President Biden and fellow leaders of NATO will come together next week extolling their remarkable unity in backing Ukraine in its war with Russia. But serious differences over the expansion of the transatlantic alliance threaten to disrupt the harmony and turn the annual summit on its head.

Cohesion among the North Atlantic Treaty Organization’s members has also been jeopardized by the war has become a seemingly interminable slog. Even Ukrainian President, Volodymyr Zelensky has said a long-anticipated counteroffensive against Russian invaders is not going as well as he hoped.

NATO’s gathering in the Lithuanian capital of Vilnius will focus world leaders on how to help nonmember Ukraine, including what kind of military aid to provide and what a longer-term security arrangement should look like – whether or not it includes future membership in the alliance. The summit comes in the wake of an aborted mutiny against Russian President Vladimir Putin by the paramilitary Wagner Group, which leaders are still analyzing.

“All eyes will be on Vilnius to see what the so-called Ukraine package will look like,” said Sean Monaghan, a visiting fellow at the Center for Strategic and International Studies who specializes in Europe.

The U.S. has already spent or pledged $75 billion to shore up the Ukrainian military and state and has authorized allies to send their U.S.-produced fighter jets into the battlefield. Biden has coaxed other European nations to pony up for the war effort, including convincing Germany to give lethal weaponry to a foreign land for the first time since World War II.

Aides say the president hopes the Vilnius summit will showcase his efforts to rebuild international partnerships that were damaged under the previous administration.

“You’ll see the NATO allies really recommitting to the basic proposition: As long as it takes,” Jake Sullivan, Biden’s national security adviser, told a small group of reporters Friday. “This will be an opportunity to really refresh the unity and the zeal that we have displayed all the way through.”

Biden will inevitably be forced to combat criticism from some Republicans – including a couple of presidential candidates – who have dismissed the importance of the Ukraine fight. They hope to galvanize a simmering impatience among segments of the public who may be tired of the expensive war. But Sullivan says the administration remains confident that it has bipartisan support – and that of the American public – for a war it has characterized as crucial to U.S. national security and the world’s rules of fair play.

It will be a struggle for summit participants to craft language that would guarantee Ukraine’s security in the coming months and years, even after the war ends. The country wants to join NATO, but that prospect is not likely while the war rages.

Foreign policy analysts instead expect NATO to lay out a series of long-term security guarantees and commitments to Ukraine’s self-defense on the sidelines of the summit until a path for membership becomes more clear. Biden has called it an “Israel-like” understanding, which would commit to a steady, open-ended flow of aid so the country can plan a long-term security strategy.

Last week, 46 foreign policy experts including Francis Fukuyama and former U.S. Rep. Tom Malinowski, D-N.J., penned an open letter in Politico Magazine calling on the alliance to use the summit to explicitly state support for Kyiv’s victory and pursuit of territorial integrity according to its 1991 borders, and to chart a course for Ukraine’s NATO membership “at the earliest achievable date.”

Former U.S. Ambassador to Ukraine John Herbst, who was among the signatories, said the more cautious approach by the administration and some European allies has overshadowed “recognition of the dangers and the opportunities of the moment.”

Though Vilnius could be a historic summit, Herbst said he doesn’t expect the alliance to rise to the occasion because of continued reticence to provoke a nuclear-armed Putin.

“The problem is, even as the administration has recognized that a Putin victory in Ukraine would be disastrous, they have been held back by their own timidity in pursuing a robust policy that would achieve the Ukrainian victory,” he said.

Also on the agenda in Vilnius is Sweden’s aspiration to become a NATO member.

After Russia’s invasion of Ukraine on Feb. 24, 2022, the historically neutral countries of Finland and Sweden declared they wanted to join NATO. Finland was quickly admitted, but Sweden stumbled over Turkey’s objections to what Ankara has said is a Swedish tolerance of Kurdish militants.

Admission to the alliance has to be unanimous, giving Turkey outsize power to block a nation like Sweden. The Nordic democracy has taken several steps in hopes of appeasing Ankara, including the extradition of a Kurdish activist wanted by Turkey and a tightening of domestic terrorism laws.

“Sweden has gone as far as it can,” said Heather Conley, president of the German Marshall Fund in Washington. Yet Turkish President Recep Tayyip Erdogan continues to refuse Sweden’s entry, dashing U.S. hopes that the matter would be settled before the Vilnius meeting.

Turkey’s obstinance is infuriating to many U.S. and European officials, some of whom have gone so far as to question the validity of Ankara’s NATO membership given the increasingly authoritarian and anti-democratic policies employed by Erdogan.

“It would be a real failure for the [NATO] alliance if it’s not able to get Sweden over the goal line here, and it’s a failure because it’s being held up by one member: Turkey,” said Max Bergmann, director of the Europe and Russia program at the Center for Strategic and International Studies. “The alliance thus far has played very nice with Turkey. … But now the rubber is sort of hitting the road here, and it really calls into question whether this is an alliance that Turkey belongs to and shares the values of.”

Sullivan reiterated that the U.S. is certain Sweden will be admitted to NATO sooner rather than later. One thing Erdogan desperately wants is to purchase F-16 fighter jets from the U.S. Congress has been holding up the sale citing various issues with Turkey.

In advance of the NATO meeting, Zelensky made a rapid-fire series of stops in Bulgaria, the Czech Republic, and Slovakia.

In Bratislava, the Slovakian capital, Zelensky again voiced Kyiv’s hopes for concrete steps toward Ukrainian membership in the alliance.

“There is strength in unity of NATO,” he told reporters.

Zelensky also traveled Friday to Turkey for a meeting with Erdogan. Zelensky said the delay over Swedish membership posed “a threat to the alliance’s strength.”

Washington has steadily ramped up the firepower of the weapons it is giving Kyiv. Ukraine’s critical need for ammunition has prompted U.S. officials to agree to supply it with cluster bombs, a controversial weapon banned in many parts of the world because of a tendency to harm civilians.

U.S. officials are aware that they will take flak at the NATO summit for the decision, which Sullivan said was not taken lightly.

“Ukraine needs the bullets so that it is not overrun,” he said.

In pleading for more weaponry, Ukrainian officials have long cited Russia’s superiority in artillery and heavy tanks, a crucial impediment as the counteroffensive gathers momentum, and have welcomed any additions to the nation’s arsenal.

“The number of weapons matters,” Mykhailo Podolyak, a key Zelensky adviser, wrote on Twitter Friday, describing the war as a struggle between lawlessness and international law. “So, weapons, more weapons, and more weapons, including cluster munitions.”

With Saturday marking the 500th day of the Russian invasion, Ukraine has made clear its frustration with the notion that the closely watched counteroffensive, which began last month, would yield swift and sweeping gains.

Its forces have been moving against deeply entrenched Russian troops along a front line stretching hundreds of miles, capturing territory mainly in small increments, with progress often measured in yards rather than miles.

On Friday, Ukraine said its troops had advanced by more than half a mile near the eastern city of Bakhmut, which fell to Russia in May.

“There are elements of the counteroffensive taking place, but not a decisive battle,” said Natalia Humeniuk, a military spokeswoman for Ukraine’s southern command in Odesa.

She said Ukraine was still inflicting losses on Russia that would eventually make it difficult for its troops to defend the territory they seized earlier in the 16-month-old invasion.

“The enemy is losing combat forces, they are losing warehouses and supply sites, they are losing logistics routes,” Humeniuk said.

But, she cautioned, not all battlefield activity by Ukraine will translate to immediate and visible territorial gains.

That may not be what NATO members in Vilnius want to hear.