BRUSSELS — Finland won final approval on Thursday to join the North Atlantic Treaty Organization after decades of nonalignment, a major shift in the balance of power between the West and Russia that was set off by the invasion of Ukraine.
The Turkish Parliament cast the last vote needed for Finland’s entry into NATO, meaning that the alliance’s border with Russia will double. It is a diplomatic and strategic defeat for President Vladimir V. Putin of Russia, who made clear when Russia invaded Ukraine that he was intent on blocking NATO’s eastward expansion.
With Finland in its fold, NATO will be in a stronger position to deter Moscow’s aggression, gaining access to a strong military, as well as Finnish airspace, ports and sea lanes. The alliance will also be better able to defend the Baltic nations and the Arctic, said Matti Pesu, a security expert with the Finnish Institute of International Affairs.
James G. Stavridis, a retired four-star American admiral and former top NATO military commander, called the move “a huge plus for NATO.”
“Geographically, their addition to the alliance adds a huge, difficult-to-defend border that complicates Putin’s calculus,” he said in an email.
Finland’s 830-mile border with Russia could present new vulnerabilities, but Finnish experts note that Russia has had to move most of its military forces away from there to sustain its war in Ukraine.
And Finland is already within range of Russian forces and nuclear-armed missiles based in the Kola Peninsula and St. Petersburg, “so Finnish membership won’t be a gamechanger,” Mr. Pesu said.
Nevertheless, the Russian Embassy in Sweden on Wednesday threatened both Finland and Sweden with military retaliation if they join NATO. Sweden’s membership application is being blocked by Turkey and Hungary
“If anyone still believes that this will somehow improve Europe’s security, you can be sure that the new members of the hostile bloc will become a legitimate target for Russia’s retaliatory measures, including military ones,” the embassy said in a posting on its Facebook page.
With the vote in the Turkish Parliament, only a bit of paperwork remains for Finland, including an exchange of letters and the placing of Finland’s accession documents, already complete, with the State Department in Washington.
Finland’s leaders decided to apply for NATO membership within weeks of Russia’s invasion of Ukraine 13 months ago. With their neighbor newly aggressive, they decided that only NATO’s commitment to collective defense could give Finland the security guarantees it needed.
For centuries Finland and Russia had conflicts. During the Winter War of 1939-1940, Finland fended off a Soviet invasion, though the Finns had to relinquish some territory and they agreed to remain formally neutral throughout the Cold War.
After the collapse of the Soviet Union, however, Finland joined the European Union and developed a partnership with NATO, dropping neutrality for military nonalignment and a close security relationship with Sweden.
That worked until Mr. Putin decided to grab Ukraine, which brought back many bitter memories in Finland of the Winter War and led to its NATO membership bid, “hand in hand” with Sweden.
Finland plans to continue pushing for rapid accession for Sweden, with which it has close military and security ties, but the Swedes have said that they accept the Finns’ decision to join alone.
It will be some time before Finland and NATO fully integrate their defense plans. Finland must still decide, for example, whether it needs or will accept foreign troops or nuclear weapons on its territory, Mr. Pesu said.
Finland is holding national elections on Sunday, and the final NATO membership decision may help the campaign of Prime Minister Sanna Marin and her Social Democrats to remain in power in a very tight race. The three largest parties appear essentially tied at less than 20 percent each in the latest poll.
In Turkey, President Recep Tayyip Erdogan is also running for re-election, with a vote scheduled for May 14. While finally agreeing on Finland’s membership to NATO, he has held off on Sweden. Mr. Erdogan claims that Sweden has become a haven for Kurdish separatists and other dissidents he considers terrorists.
Stockholm has made efforts to placate him, including adopting a new antiterrorism law. The NATO secretary general, Jens Stoltenberg, has said that Stockholm has fulfilled the commitments it made to Turkey at the last NATO summit in Madrid.
But Mr. Erdogan has not budged.
His tough stance on the Kurds is popular in Turkey, and he has benefited by displaying international influence and leverage within NATO, and as a potential mediator between Ukraine and Russia. Mr. Erdogan announced this week that Mr. Putin may visit Turkey on April 27 for the inauguration of the country’s first nuclear power reactor, which was built by Russia’s state nuclear energy company, Rosatom — an event clearly timed to help his political campaign.
Unlike nearly all European countries, Finland did not shrink its military after the collapse of the Soviet Union. So it brings a lot to the table for a country of only some 5.6 million people.
The number of active-duty military personnel in Finland’s defense forces is a modest 23,000 troops, but its wartime strength can grow quickly to 280,000 because of an extensive conscription system.
And Finland’s artillery forces are the largest and best-equipped in Western Europe, with some 1,500 artillery weapons, including 700 howitzer guns, and 700 heavy mortar and 100 rocket launcher systems, according to an analysis by the Wilson Center, a Washington-based research organization.
A major designer of icebreaker ships, Finland will also play a significant role conducting maritime operations in the increasingly contested Arctic region, officials said.
Hungary, too, has blocked Sweden’s application. The Hungarian leader, Prime Minister Viktor Orban, has warm relations with Mr. Putin and, like Turkey, continues to get Russian gas and oil.
Mr. Orban has in the past said that he supports Sweden’s bid, but a statement on Wednesday from his spokesman, Zoltan Kovacs, objected to what he said was Sweden’s “openly hostile stance” toward Hungary and Turkey. The Hungarian government understands why Sweden seeks to be a part of the alliance, Mr. Kovacs said, but “there is an ample amount of grievances that need to be addressed before the country’s admission is ratified.”
Hungarian officials have complained that Sweden has been particularly critical of Hungary’s constitutional and legal changes and their threat to the rule of law, though most member states of the European Union have raised similar concerns.
The Turkish vote clears the way for Turkey to send an acceptance letter to the United States to be filed at the State Department alongside Finland’s acceptance letters from the other 29 NATO countries, in accordance with Article 10 of NATO’s founding treaty.
The State Department will then notify Mr. Stoltenberg that the conditions have been met for Finland to become a member of NATO.
Eric Schmitt contributed reporting from Washington, and Ben Hubbard from Istanbul.
