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.

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.

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.

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.

In a positive sign for Ukraine's efforts to join NATO, Ukrainian Foreign Minister Dmytro Kuleba said that the alliance has agreed to let Kyiv bypass a detailed formal process in its application to join the alliance. Kuleba said in a tweet Monday that "following intensive talks, NATO allies have reached consensus on removing MAP [Membership Action Plan] from Ukraine's path to membership."

But 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.

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 national security adviser Jake 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.

"I do not think you will see fracture, division, or disunity ... as a result of this decision. Even though many allies the signatories to [the Oslo Convention] are in a position where they themselves cannot say, 'We are for cluster munitions.' But we have heard nothing from people saying this cast doubt on our commitment, this cast doubt on coalition unity, or this cast doubt on our belief that the United States is playing a vital and positive role as leader of this coalition in Ukraine," he said.

As the day begins, Biden will participate in an official arrival ceremony and bilateral meeting with the summit's host, Lithuania President Gitanas Nausėda. 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.