President Biden and Ukraine’s President Volodymyr Zelensky speak during a meeting on the sidelines of the NATO summit in Vilnius, Lithuania, on Wednesday. Susan Walsh/Associated Press

VILNIUS, Lithuania — President Biden pledged Wednesday that Western allies “will not waver” in defense of Ukraine, casting the struggle against Russian aggression as one of the world’s central challenges requiring a broad coalition of countries to stand in defense of freedom.

“Our unity will not falter,” Biden declared. “I promise you.”

He made the promise at the NATO summit in the capital of Lithuania, a country that he said knows the “transformational power of freedom” after spending decades under Moscow’s thumb. He drew parallels between Lithuania’s struggle to escape Soviet rule and Ukraine’s ongoing fight to repel Russia’s deadly invasion, highlighting the importance of rallying allies to take on the challenge.

“America never recognized the Soviet occupation of the Baltic,” he said to an outpouring of cheers from a crowd of thousands in a courtyard at Vilnius University draped with American and Lithuanian flags. “Never, never.” More spectators gathered in an overflow area, where a big screen was set up.

Biden spent two days in Vilnius for the annual NATO summit, where members of the Western military alliance pledged more support for Ukraine but stopped short of extending an invitation for the besieged country to join the alliance. After meeting with Ukrainian President Volodymyr Zelensky, who entered the summit demanding a clear path for his country toward joining the alliance, something that was initially promised back in 2008, Biden said that other security guarantees agreed to at the summit will be even more significant.

“One thing Zelensky understands now is that whether or not he’s in NATO now, it’s not relevant as long as he has commitments,” Biden said, comparing the situation to how the U.S. ensures Israel’s security edge over its neighbors.

The president pointed to the U.S. and allied response to Moscow’s invasion as a model for how to respond to other global challenges, from climate change to the rise of China, saying nations’ positions are stronger when they “build the broadest and deepest coalition.”

President Biden addresses the public during an event at Vilnius University on the sidelines of a NATO summit in Vilnius, Lithuania, on Wednesday. Mindaugas Kulbis/Associated Press

“Our commitment to Ukraine will not weaken,” he said. “We will stand for freedom today, tomorrow, and for as long as it takes.”

The president was headed next to Finland, the newest member of NATO, for a meeting of Nordic leaders. During his speech, Biden hailed an agreement to advance Sweden’s membership in NATO after Turkish President Recep Tayyip Erdogan agreed to drop his objections.

“President Erdogan kept his word,” Biden said, clearing a path for the alliance to have 32 members.

The U.S. president’s enthusiasm for expanding NATO has not extended to Ukraine. He’s expressed concerns about the country’s readiness to join the alliance, as well as fears that the West could be drawn into a wider conflict with Russia.

The competing priorities amid Europe’s bloodiest war in generations created an undercurrent of friction even as Biden and Zelensky projected a united front when they met earlier Wednesday. Their public encounter had the vibe of two leaders clearing the air, and each conspicuously heaped praise on his counterpart.

Biden lauded Zelenskyy and Ukrainians for their courage by saying it’s “been a model for the whole world to see.” Zelensky thanked Biden and the American people for billions of dollars in military assistance, saying that “you spend this money for our lives.”

Wearing a blue-and-yellow-striped tie in the colors of the Ukrainian flag, Biden acknowledged that Zelensky has occasionally been unsatisfied by unfulfilled requests for weapons.

“The frustration, I can only imagine,” Biden said. “I know that you’re many times frustrated about whether things get to you quickly enough, what’s getting to you, and how we’re getting it. But I promise you, the United States is doing everything we can to get you what you need.”

Biden also said the war had created a sense of unity about opposing international aggression.

“It’s bringing the world together,” he said. “It’s a hell of a price to pay, but it’s bringing the world together.”

The meeting came after a few other encounters between Biden and Zelensky at the summit. They sat close to each other at the inaugural meeting of the NATO-Ukraine Council, a new forum that’s intended to give Kyiv a greater voice within the alliance.

And they shared the stage as the Group of Seven, which includes the world’s most powerful democratic countries, announced plans for long-term security assistance for Ukraine.

But Wednesday afternoon was the first opportunity for Biden and Zelensky to sit down privately with their advisers after their public comments.

And by then, Zelensky had softened his tone considerably. En route to Vilnius on Tuesday, he had blasted NATO’s vague plans for Ukraine’s eventual membership, tweeting, “It’s unprecedented and absurd when a time frame is set neither for the invitation nor for Ukraine’s membership.”

Jake Sullivan, Biden’s national security adviser, said everyone “needs to look squarely at the fact” that allowing Ukraine to join NATO at this point “means war with Russia.”

“That is an inescapable fact,” he told CNN.

Sullivan credited Biden with ensuring that NATO is “more unified and more determined and more decisive than at any point.”

“That’s President Biden’s legacy when it comes to NATO, and it’s one that he can be very proud of,” he said.

In an interview with the Associated Press, before Biden left on his trip, Senate Minority Leader Mitch McConnell said the president has been “heading in the right direction but not fast enough” when it comes to supporting Ukraine.

“The weapons transfers never seem to happen as soon as they’re announced,” said McConnell, a Kentucky Republican. Although Ukrainians are “extremely grateful for the help,” he said, the assistance “frequently doesn’t get there soon enough to be the most effective.”

Although McConnell has been a firm supporter of sending help to Ukraine, other Republicans have voiced skepticism, creating uncertainty about Biden’s ability to make long-term financial commitments.