With a simple exchange of documents, Finland on Tuesday became NATO’s 31st member state, a strategic defeat for President Vladimir V. Putin of Russia, who was determined to block the alliance’s expansion but instead galvanized Finland to join amid Moscow’s devastating war in Ukraine.
Later in the day, Finland’s national flag was raised at NATO headquarters, a deeply symbolic moment and a stark display of the shifting global dynamics, as the West shores up its allegiances in response to Russia’s aggression toward its neighbor. With Finland’s membership assured, NATO doubled its borders with Russia and gained access to a strong military with a deep history of countering Russia.
NATO’s commitment to collective defense will now extend to a country that shares an 830-mile border with Russia and was twice invaded by its neighbor in the 20th century. If Finland is attacked, it can call on all members of the alliance for aid, a psychological and practical boost to Finns’ sense of security.
Now that Finland is a full-fledged member, said NATO’s secretary general, Jens Stoltenberg, “we are removing the room for miscalculation in Moscow about NATO’s readiness to protect Finland, and that makes Finland safer and stronger, and all of us safer.”
President Sauli Niinisto of Finland, who attended the ceremony on the North Atlantic Treaty Organization’s 74th anniversary, declared, “It is a great day for Finland.”
Russia tried to restrict Finns’ freedom of choice and “tried to create a sphere around them,” he added. “We are not a sphere.”
Many details about how Finland will integrate into the alliance are still to be worked out. Among the pressing issues at hand, a new Finnish government, still to be negotiated after an election on Sunday, must decide whether the country will accept foreign troops on its soil, or even nuclear weapons belonging to its allies.
But Finland’s membership adds one of Western Europe’s most potent militaries to the alliance, as well as intelligence and border-surveillance abilities, U.S. officials say. Finland’s artillery forces are the largest and best equipped among European NATO members, with 1,500 artillery, including 700 howitzer, 700 heavy mortars and 100 rocket launcher systems, according to an analysis published last year.
NATO will eventually have access to Finnish ports, air space and sea lanes, which will greatly enhance its defensive capabilities.
“Geographically, their addition to the alliance adds a huge, difficult-to-defend border that complicates Putin’s calculus,” James G. Stavridis, a retired four-star U.S. admiral and former NATO military commander, has said. “A huge plus for NATO.”
President Biden called Finland’s accession “the fastest ratification process in NATO’s modern history.”
“When Putin launched his brutal war of aggression against the people of Ukraine, he thought he could divide Europe and NATO,” Mr. Biden said in a statement. “He was wrong. Today, we are more united than ever. And together — strengthened by our newest ally Finland — we will continue to preserve trans-Atlantic security, defend every inch of NATO territory and meet any and all challenges we face.”
Wary of Moscow’s ire, Finland and Sweden remained militarily nonaligned even after the collapse of the Soviet Union. But last year they applied to join NATO, after Russian forces swept across the border into neighboring Ukraine. In response, Russia said it would bolster its defenses near its long border with Finland.
In Moscow on Tuesday, Dmitry S. Peskov, the Kremlin spokesman, called Finnish membership in NATO an “assault” on Russia’s security and national interests. “This forces us to take countermeasures in tactical and strategic terms,” he said.
Finland’s entry into the alliance was unlocked when Turkey, the last of the holdouts among the body’s 30 member states, ratified the application in a parliamentary vote last week. Finland had wanted to join “hand in hand” with Sweden, but that process was upended because Turkey and Hungary objected.
Turkey’s president, Recep Tayyip Erdogan, who is up for re-election on May 14, angrily questioned the depth of Sweden’s commitment to fighting terrorism. Mr. Erdogan has called for Sweden to extradite figures he regards as terrorists, including Kurds and others he believes supported the 2016 coup attempt against him.
At the ceremony on Tuesday, Mr. Stoltenberg and Mr. Niinisto said they would continue to press Turkey and Hungary to ratify Sweden’s membership as soon as possible.
Stockholm has said it is not sure it would be able to join the alliance in time for a planned NATO summit in July, but Mr. Stoltenberg expressed confidence that Sweden would join before then, and announced publicly that he had invited Ukraine’s president, Volodymyr Zelensky, to come in person to the summit in Lithuania.
Finland’s journey to NATO involved dueling ceremonies — and a bit of paperwork.
In Brussels, the Belgian capital, Finland’s national flag, bearing a blue Nordic cross on a white field, took its place on Tuesday between the emblems of Estonia and France as the Finnish anthem played.
Protocol also dictated an exchange of letters and the placing of Finland’s accession documents with the U.S. State Department, represented by Secretary of State Antony J. Blinken, who was in Brussels.
The formalities were at once outwardly mundane but weighted with significance: When Turkey’s and Finland’s representatives handed over their official texts to Mr. Blinken, Finland’s membership was sealed.
“With receipt of this instrument of accession, we can now declare that Finland is the 31st member of the North Atlantic Treaty,” Mr. Blinken said.
In a statement, Mr. Blinken said, “Finland is stronger and safer within the alliance, and the alliance is stronger and safer with Finland as its ally.” He also urged Turkey and Hungary “to ratify the accession protocols for Sweden without delay, so we can welcome Sweden into the alliance as soon as possible.”
The ceremony occurred as foreign ministers from the alliance were gathering in Brussels for a two-day meeting. Finland’s foreign minister, Pekka Haavisto, attended — his country’s first time as a full-fledged member. The alliance convened a NATO-Ukraine Commission, a more formal meeting previously blocked by Hungary.
That focused on how to help Ukraine wage a counteroffensive against Russian troops, expected to begin in late spring or early summer, and how to intensify Ukraine’s partnership with NATO, Mr. Stoltenberg said.
The foreign ministers also discussed how to accelerate Ukraine’s transition to NATO-compatible military equipment and ammunition. Ukraine’s foreign minister, Dmytro I. Kuleba, along with Sweden’s foreign minister, Tobias L. Billstrom, also attended.
On Tuesday, the United States announced it was sending Ukraine additional air defense missiles and other munitions as part of a $2.6 billion aid package to help Kyiv prepare for an expected spring offensive. The package includes $500 million in ammunition and equipment from U.S. military stockpiles, and $2.1 billion that the United States will use to buy munitions, radar and other weapons to send to Ukraine in the future.
“Russia alone could end its war today,” Mr. Blinken said in a statement. “Until Russia does, the United States and our allies and partners will stand united with Ukraine for as long as it takes.”
Full Finnish integration into NATO’s command structure will take some time, and negotiations to create a new coalition government in Helsinki after the vote on Sunday are expected to take weeks.
Finland must learn to think collectively and adapt its military strategy appropriately, said Matti Pesu, a security expert at the Finnish Institute of International Affairs. It must also think through its nuclear strategy and its views toward nuclear arms control now that it will be covered by the American nuclear umbrella.
The country will deepen its defense relationships with countries like Britain, Estonia, and Norway, Sweden and the United States, he said. And “its Russia policy will increasingly become deterrence-based” in the collective alliance, rather than depending on its own resilience.
“At times like this,” Mr. Stoltenberg, NATO’s secretary general, said on Tuesday, “friends and allies are more important than ever, and Finland now has the strongest friends and allies in the world.”
Eric Schmitt and Helene Cooper contributed reporting from Washington.