Sweden on Thursday formally joined NATO as the 32nd member of the transatlantic military alliance, ending decades of post-World War II neutrality as concerns about Russian aggression in Europe have spiked following Russia's 2022 invasion of Ukraine.

Swedish Prime Minister Ulf Kristersson and Secretary of State Antony Blinken presided at a ceremony in which Sweden's “instrument of accession” to the alliance was officially deposited at the State Department.

“This is a historic moment for Sweden. It's historic for the alliance. It's history for the transatlantic relationship,” Blinken said. “Our NATO alliance is now stronger, larger than it's ever been.”

Kristersson wrote in a social media post that “we are therefore a safer country.”

Later Thursday. Kristersson will visit the White House and then be a guest of honor at President Joe Biden's State of the Union address to Congress.

The White House said that having Sweden as a NATO ally “will make the United States and our allies even safer.”

"NATO is the most powerful defensive alliance in the history of the world, and it is as critical today to ensuring the security of our citizens as it was 75 years ago when our alliance was founded out of the wreckage of World War II,” it said in a statement.

NATO Secretary-General Jens Stoltenberg described it as “a historic day.”

“Sweden will now take its rightful place at NATO's table, with an equal say in shaping NATO policies and decisions,” he said in a statement.

The Swedish flag will be raised outside the military organization's headquarters in Brussels on Monday. Stoltenberg underscored that the Nordic country “now enjoys the protection granted under Article 5, the ultimate guarantee of allies' freedom and security.”

Article 5 of NATO's treaty obliges all members to come to the aid of an ally whose territory or security is under threat. It has only been activated once – by the U.S. after the Sept. 11, 2001, attacks – and is the collective security guarantee that Sweden has sought since Russia invaded Ukraine.

“Sweden's accession makes NATO stronger, Sweden safer and the whole alliance more secure,” Stoltenberg said. He added that the move “demonstrates that NATO's door remains open and that every nation has the right to choose its own path.”

Sweden, along with Finland, which joined NATO last year, both abandoned long-standing military neutrality that was a hallmark of the Nordic states' Cold War foreign policy after Russia invaded Ukraine in early 2022.

Biden, in his speech to Congress, is expected to cite Sweden's accession to NATO as evidence that Russian President Vladimir Putin's intent to divide and weaken the alliance has failed as a direct result of the Ukraine invasion. And, the Democratic president is expected to use Sweden's decision to join to step up calls for reluctant Republicans to approved stalled military assistance to Ukraine as the war enters its third year.

Biden and his NATO counterparts have vowed that Ukraine will join one day, too.

Sweden's membership had been held up due to objections by NATO members Turkey and Hungary. Turkey expressed concern that Sweden was harboring and not taking enough action against Kurdish groups that it regards as terrorists, and Hungary's populist President Viktor Orban has shown pro-Russian sentiment and not shared the alliance's determination to support Ukraine.

After months of delay, Turkey ratified Sweden's admission earlier this year, and Hungary did so this week.