By Betsy Klein, CNN

Vilnius, Lithuania (CNN) — President Joe Biden and alliance leaders enter the first day of the high-stakes NATO Summit Tuesday with a reinvigorated sense of unity after a major win on Monday evening when Turkey agreed to Sweden’s bid to join the alliance.

“Thank you, Mr. President, for hosting this historic summit at an important time. The first time that NATO leaders will meet, 31, together, and looking forward to meeting very soon with 32 members with the addition of Sweden,” Biden said as he began a bilateral meeting with summit host Lithuanian President Gitanas Nausėda.

“I am confident we’ll get that done,” Biden added.

The leaders gather here in Vilnius, Lithuania, for a two-day summit that could become one of the most consequential gatherings for the alliance in modern history, coming about a month into Ukraine’s slow counteroffensive and weeks after a failed mutiny in Russia became a major threat to President Vladimir Putin’s leadership.

While national security experts had warned that a failure to admit Sweden to NATO could portend cracks in the alliance, Monday evening’s announcement – which came just hours after Biden landed in the Lithuanian capital city – marks a stunning about-face from Turkish President Recep Tayyip Erdoğan, who has blocked the accession bid for more than a year.

Biden and other members have touted unprecedented unity among the alliance in the face of Russia’s war, and the move also provides leaders a significant show of force going into the summit.

“We’re coming into this consequential summit with a full head of steam,” national security adviser Jake Sullivan told reporters in Vilnius Tuesday morning.

“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,” he added.

Sullivan said Turkey’s decision was “the product of direct talks among the three parties to that agreement,” but the US “had significant recent engagement with everyone involved,” pointing to Biden’s hosting of Swedish Prime Minister Ulf Kristersson at the White House last week, his call to Erdoğan on Sunday, engagement with Secretary of State Antony Blinken with his Swedish and Turkish counterparts, and his own engagement with his counterparts.

He pointed to questions about unity in the past week as he warned that the Vilnius summit will “very much disappoint” Russian President Vladimir Putin.

“In a lot of the coverage coming into the summit, I would say, rumors of the death of NATO’s unity were greatly exaggerated. Every few months, the question is called: Can the West hang together? Can NATO hang together?” Sullivan said.

“Every time allies gather, that question gets re-upped, and every time the allies come together and answer it forcefully and vehemently: ‘Yes we can.’ Vladimir Putin has been counting on the West to crack, NATO to crack, the Transatlantic Alliance to crack. He has been disappointed at every turn.”

The Swedes will not join the alliance right away – it will take a parliamentary procedure in Turkey to formally approve their membership and Hungary must also drop their objections, which it is expected to do now that Erdoğan is on board.

Biden called for “swift ratification” in a statement following the news, and the US, Sullivan said Tuesday, looks forward to “welcoming Sweden as NATO’s 32nd ally in the near future.”

‘United, positive signal’

There will be other critical matters for the US president to address in Vilnius this week, including his controversial decision to send cluster munitions to Ukraine – which are banned by more than 100 nations, including some key US allies. There are also questions for leaders about a pathway for Ukraine to eventually join NATO, and the possibility of additional security assistance, with President Volodymyr Zelensky expected to attend in person on Wednesday and hold an in-person meeting with Biden.

The meeting will mark yet another sign of unity as Zelensky’s attendance at the summit had been in question. Russia’s war in Ukraine is among the top agenda items for NATO leaders along with discussing a future pathway for the war-torn country to join the alliance, which has prompted some division among leaders.

Biden said Tuesday that he agreed on proposed language for Ukraine’s future ability to join NATO, comments that came moments after Zelensky issued a blistering statement about that expected language, suggesting it did not go far enough toward his accession goals. In an interview with CNN’s Fareed Zakaria last week, Biden said that Ukraine is not yet ready to enter NATO, saying that Russia’s war in Ukraine needs to end before the alliance can consider adding Kyiv to its ranks.

“We agree on the language that we proposed – and you proposed relative to the future of Ukraine being able to join NATO. We’re looking for a continued, united NATO,” Biden said in brief remarks alongside NATO Secretary General Jens Stoltenberg at the summit site.

Zelensky said in a tweeted statement moments earlier that he has “received signals that certain wording is being discussed without Ukraine,” emphasizing that the “wording is about the invitation to become NATO member, not about Ukraine’s membership.”

“It’s unprecedented and absurd when time frame is not set neither for the invitation nor for Ukraine’s membership. While at the same time vague wording about ‘conditions’ is added even for inviting Ukraine. It seems there is no readiness neither to invite Ukraine to NATO nor to make it a member of the Alliance,” Zelensky said, adding, “Uncertainty is weakness. And I will openly discuss this at the summit.”

Sullivan declined to predict a specific date for Ukraine joining the alliance.

“I can’t put a timetable on it. I don’t believe that you will see that coming out of here,” he said. “From our perspective, it is the work of the alliance with Ukraine to lay out that reform path, and then to have Ukraine work towards it.”

Article 5 commitment

The summit also comes days after the US announced that it will be sending cluster munitions to Ukraine for the first time, a move aimed at bolstering Ukraine’s offensive capabilities that has prompted some public disagreement from allied countries, a move that Biden called a “difficult decision” in his interview with Zakaria but was necessary because Ukraine is running low on ammunition.

United Kingdom Prime Minister Rishi Sunak noted to reporters over the weekend that the UK is “signatory to a convention, which prohibits the production or use of cluster munitions and discourages their use.”

But earlier in the week Sullivan sought to downplay any concern that Biden’s decision to send cluster munitions would present any “fracture” with allied countries that oppose the use of such equipment and said the US has not gotten any negative feedback from allies since the announcement. And on Tuesday, he emphasized that the move is temporary.

“We view that as temporary because many months ago, we began the intensive process of ramping up our unitary round production. Once it hits a level where unitary round production can satisfy Ukraine’s needs, then there will be no need to continue giving cluster munitions,” he said, declining to provide a timeline due to questions of usage rates and the defense industrial base hitting its production marks.

A unitary round only has one explosive charge in it, as compared to cluster munitions that are canisters that carry tens to hundreds of smaller bomblets.

Sullivan added, “We were not prepared to leave Ukraine defenseless, period. So for us, when it came down to the choice, our choice was, despite the difficulty, despite the challenges, despite the risks of civilian harm associated with cluster munitions, the risk to civilian harm of leaving Ukraine without the ammo it needed was, from our perspective, greater.”

As the day began Tuesday, Biden participated in an official arrival ceremony and bilateral meeting with Nausėda, where he reaffirmed the US’ “commitment to NATO” and to Article 5, the principle that an attack on one member of NATO is an attack on all members.

“We take – NATO takes – all of us take Article 5 literally. One inch of NATO territory means we’re all – all on board together against whoever is violating that space. We will defend every inch of it,” he said.

Biden is also meeting with members of of the US Congress in attendance at the summit, Sullivan said.

He will also participate in an official greeting with Nausėda and NATO Secretary General Jens Stoltenberg, who recently agreed to extend his term an additional year. NATO leaders will participate in a family photo before their first meeting gets underway. And later Tuesday, Biden will hold a bilateral meeting with Erdoğan on the sidelines of the summit, where the two are expected to discuss efforts to “(enhance) defense and deterrence in the Euro-Atlantic area,” per Biden’s statement.

Biden arrived in Vilnius on Monday evening following a meeting with Sunak at No. 10 Downing Street and an engagement on climate change with King Charles III at Windsor Castle, marking the president’s first meeting with the monarch since his coronation.