Sweden formally joined NATO on Thursday, becoming its 32nd member two years after Russia’s invasion of Ukraine and sharply bolstering, with Finland, the military alliance’s deterrent in the Baltic and North Seas.
With the addition of the new Nordic member states — Finland joined last year — the president of Russia, Vladimir V. Putin, now finds himself faced with an enlarged and motivated NATO, one that is no longer dreaming of a permanent peace but instead facing years of trying to contain a newly aggressive, imperial Russia.
On Thursday, after months of uncertainty caused by the hesitations of Turkey and Hungary, Sweden officially became a member by depositing its legal paperwork — its instrument of accession to the North Atlantic Treaty — with the U.S. State Department in Washington.
In a brief ceremony, Secretary of State Antony J. Blinken received the documents from Ulf Kristersson, the Swedish prime minister, and said: “Good things come to those who wait.” Mr. Blinken said that “everything changed” after Russia’s invasion. “Swedes realized something very profound: that if Putin was willing to try to erase one neighbor from the map, then he might well not stop there.”
Mr. Blinken said Sweden’s membership was a clear example “of the strategic debacle that Ukraine has become for Russia,” adding: “Everything that Putin sought to prevent, he has actually precipitated by his actions, by his aggression.”
Mr. Kristersson said that “today is a truly historic day.” Sweden, he said, “will defend freedom together with the countries closest to us — both in terms of geography, culture and values.” He pledged that Sweden, which had largely dismantled its ground forces after 1989 but has maintained a powerful air force and navy, would soon reach NATO’s goal of spending 2 percent of G.D.P. on the military.
NATO is planning a ceremony on Monday to raise the Swedish flag at its Brussels headquarters, as well as at NATO commands across Europe and North America.
Jens Stoltenberg, the NATO secretary general, called it “a historic day.” In a statement, he said that “Sweden will now take its rightful place at NATO’s table, with an equal say in shaping NATO policies and decisions. After over 200 years of nonalignment Sweden now enjoys the protection granted under Article 5, the ultimate guarantee of allies’ freedom and security.”
Sweden, he continued, “makes NATO stronger, Sweden safer and the whole alliance more secure.”
The Russian government has said that it will now take undefined measures to enhance its own defense against the newly enlarged NATO, which has, with Sweden and Finland, a much longer land border with Russia than before.
“Sweden brings predictability, removing any uncertainty about how we would act in a crisis or a war,” said Robert Dalsjo, director of studies at the Swedish Defense Research Agency. Given Sweden’s geography, including Gotland, the island that helps control the entrance to the Baltic Sea, membership “will make defense and deterrence much easier to accomplish,” he said.
After Russia’s full-scale invasion two years ago, Finland, with its long border with Russia, saw the most imminent danger. The Swedes did, too, but were also convinced, especially on the political left, by a sense of moral outrage that Russia, a permanent member of the U.N. Security Council, would seek to destroy a peaceful, sovereign neighbor.
“Overall the feeling is that we’ll be safer,” said Anna Wieslander, a Swede who is director for northern Europe for the Atlantic Council.
With Sweden and Finland together in NATO, it will be much easier to bottle up the Russian surface navy in the Baltic Sea and to monitor the High North. Russia still has up to two-thirds of its second-strike nuclear weapons there, based on the Kola Peninsula.
Sweden, with its own advanced high-tech defense industry, makes its own excellent fighter planes, naval corvettes and submarines, designed to operate in the difficult environment of the Baltic Sea. It has already begun to develop and build a new class of modern submarines and larger corvettes for coastal and air defense.
With NATO membership, it will be easier now to coordinate with Finland and Denmark, which also have key islands in the Baltic, and with Norway.
Sweden may also join NATO’s multinational forward brigade in Latvia, intended to put allied troops in all the alliance countries bordering Russia.
Sweden’s main tasks, Ms. Wieslander said, will be to help guard the Baltic and the airspace over Kaliningrad; to ensure the security of Gothenburg, which is key for resupply and reinforcements; and to serve as a staging area for American and NATO troops, with agreements for the advance positioning of equipment, ammunition, supplies and field hospitals.