BRUSSELS — Sweden’s flag was raised at NATO headquarters on Monday, cementing the Nordic country’s place as the 32nd member two years after Russia’s full-scale invasion of Ukraine persuaded its reluctant public to seek safety under the alliance’s security umbrella.

Under a steady rain, Swedish Prime Minister Ulf Kristersson, Crown Princess Victoria and NATO Secretary-General Jens Stoltenberg looked on as two soldiers raised the blue banner emblazoned with a yellow cross among the official circle of national flags at the alliance's headquarters in Brussels.

“The Russian, brutal, full-scale invasion against Ukraine united Sweden behind the conclusion that a full-fledged NATO membership is the only reasonable choice,” Kristersson said. Swedish government ministers and party leaders from across the political spectrum attended in a show of national unity.

Sweden set aside decades of post-World War II neutrality when it formally joined NATO on March 7. The neighboring nation of Finland had already joined in April 2023 in another historic move ending years of military nonalignment.

Finland's defense ministry welcomed “our brothers and sisters in arms,” saying on X, formerly Twitter, that “now we stand at the beginning of a new era. Together and with other allies in peace, in crisis and beyond.”

Russian President Vladimir Putin’s decision to order troops into Ukraine on Feb. 24, 2022, triggered an about-face in public opinion in both countries and within three months they had applied to join the world’s biggest security organization.

Putin claimed to have launched the war, at least in part, over NATO’s eastward expansion toward Russia. But the war had an adverse effect, pushing more countries to join the alliance. NATO leaders have promised that Ukraine itself will join one day, although almost certainly not while the conflict rages on.

“When President Putin launched his full-scale invasion two years ago, he wanted less NATO, and more control over his neighbors. He wanted to destroy Ukraine as a sovereign state, but he failed,” Stoltenberg said.

“NATO is now bigger and stronger. Ukraine is closer to NATO than ever before, and as the brave Ukrainians continue to fight for their freedom, we stand by their side,” he said.

Sweden’s membership completes a strategic ring of NATO territory around the Baltic Sea. The country now benefits from the alliance’s collective security guarantee — Article 5 of its treaty — a vow that an attack on one of them will be met by a response from them all.

“We have chosen you, and you have chosen us. All for one, and one for all,” Kristersson said, and he vowed that his country would uphold the values enshrined in NATO’s founding Washington Treaty.

The flag-raising ceremony came as 20,000 troops from 13 countries conduct NATO drills in the high north of new member Sweden as well as its neighbors Finland and Norway.

The Nordic drill is part of wider exercises called Steadfast Defender 24, NATO’s largest in decades, with up to 90,000 troops taking part over several months to show any adversary that the alliance can defend all of its territory from North America up to its borders with Russia.

“We are humble, but we are also proud. We know the expectations for Sweden are high, but we also have high expectations for ourselves,” Kristersson told reporters minutes before the ceremony. “We will share burdens, responsibilities and risks with our allies.”

Sweden brings to the table well-trained and -equipped armed forces. The country has been working in close partnership with NATO during military exercises over the years, and even more since Russia's all-out war began. Sweden also meets NATO's defense spending target of 2% of gross domestic product.