The blue and yellow flag of Sweden was hoisted at NATO headquarters in Brussels, symbolising the country's becoming the newest - and 32nd - member of the alliance, AFP reports.
NATO chief Jens Stoltenberg and Swedish Prime Minister Ulf Kristersson presided over the flag-raising ceremony, which took place in the rain as a military band played the alliance's anthem.
"Sweden joining NATO is a historic event and shows that Russian President Vladimir Putin has failed in his attempt to weaken the alliance," Stoltenberg said.
"Not only has the Kremlin's invasion forced the formerly non-aligned states of Sweden and Finland to come under the NATO defense umbrella, but Ukraine is now closer to NATO membership than ever before. 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," he added.
"NATO is bigger and stronger," he said.
Finland joined NATO last year, quickly after applying.
Sweden took longer to join as NATO members Turkey and Hungary delayed the process. But in January Ankara and last week Budapest finally gave their formal consent.
Sweden's formal accession took place last March 7 in Washington, where Kristerszohn handed over the relevant documents to the US State Department. He then attended President Joe Biden's State of the Union speech.
During the rain-soaked Swedish flag-raising ceremony in front of the NATO building, the historic enlargement was symbolically sealed.
The flag - a yellow cross on a blue background - was raised between the flags of Spain and Turkey, arranged in alphabetical order.
"Sweden's accession shows once again that NATO's door remains open. No one can close it. Every nation has the right to choose its own path," Stoltenberg said just before its hoisting.
At a joint media conference with Kristersson minutes earlier, Stoltenberg had admitted that he had "not expected" the accession of Finland and Sweden during his tenure as secretary general of the alliance.
"Of course, that changed completely with the full-scale invasion of Ukraine, and since then things have really developed very quickly," he said.
Kristersson said Sweden would now "share the burdens, the responsibilities - and the risks - with our allies."
"The security situation in our region has not been this serious since World War II and Russia will remain a threat to Euro-Atlantic security for the foreseeable future," he added.
For Russians, Putin frames his 2022 all-out invasion of Ukraine as a defensive "special military operation" against an expanding NATO.
Alliance member states have provided military and financial support to Kiev in its fight.
But the pace is slowing as U.S. political will cracks ahead of November's presidential election and Europe struggles to meet the ammunition needs of a superior Ukraine.
Stoltenberg has said a negotiated peace is possible for Ukraine, but only if Putin withdraws his forces.
"President Putin started this war and he can end it today. But Ukraine does not have that option. Surrender is not peace. We must continue to strengthen Ukraine to show President Putin that he will not get what he wants on the battlefield, but must sit down and negotiate a solution," the NATO chief said. / BGNES
