BRUSSELS — Even before his invasion of Ukraine, President Vladimir V. Putin of Russia had warned Sweden and Finland of “retaliation” should they join NATO. It was, after all, Ukraine’s desire to join the alliance that he cited time and again as provocation for his war.
But if his invasion of Ukraine has succeeded at anything so far, it has been to drive the militarily nonaligned Nordic countries into the arms of NATO, as Russian threats and aggression heighten security concerns and force them to choose sides.
In a rapid response to Russia’s invasion — and despite Mr. Putin’s threat of “serious political and military consequences” — both Finland and Sweden are now seriously debating applications for membership in the alliance and are widely expected to join.
Their accession would be another example of the counterproductive results of Mr. Putin’s war. Instead of crushing Ukrainian nationalism, he has enhanced it. Instead of weakening the trans-Atlantic alliance, he has solidified it. Instead of dividing NATO and blocking its growth, he has united it.
“With the contours of European security irrevocably altered since Russia’s invasion of Ukraine, the direction of thinking in both countries — especially Finland — is getting clearer by the day,” wrote Anna Wieslander and Christopher Skaluba of the Atlantic Council.
“From Moscow’s perspective, the result might be another unwanted consequence of its needless and reckless aggression,” they said.
At a news conference in Stockholm on Wednesday with Prime Minister Magdalena Andersson of Sweden, Prime Minister Sanna Marin of Finland said a decision on whether to apply for membership would be made “within weeks” as her government submitted a document to inform parliamentary debate on the issue.
“There are of course pros and cons with being a member of NATO, as there are pros and cons of other security choices,” Ms. Andersson said. But, she added, “I see no point in delaying this analysis or the process” over whether to join.
NATO officials are publicly discreet, saying only that the alliance has an open-door policy and any country that wishes to join can ask for an invitation. After a meeting of alliance foreign ministers last week, the secretary general, Jens Stoltenberg, was coy, but said: “There are no other countries that are closer to NATO.”
Even a speedy application process could take a year, raising concerns that the two countries would be vulnerable to Russia in that time.
Already, the alliance is debating what kind of security guarantees could be provided to Finland and Sweden while their membership is considered. The goal would be to try to ensure that Russia or any other adversary did not take advantage of the interim before the two countries were part of NATO and could benefit from its promise of collective defense.
Both Sweden and Finland are members of the European Union and already have strong partnerships with NATO, participating in military exercises and even strategic and operational planning.
But Finland, with its long border with Russia, famously survived the Cold War as an independent and unoccupied democracy by studiously hewing to neutrality, something some have suggested for Ukraine. Finland has sided openly with the West since the Soviet Union collapsed, though it has kept, like Sweden, a policy of military nonalignment.
Mr. Putin’s invasion has led to a head-spinning turnaround in public opinion in Finland in favor of joining the alliance. Led quietly by its president, Sauli Niinisto, Finland is clearing the path toward NATO membership for a more reluctant Sweden.
While Finland’s security doctrine includes an option to join NATO if circumstances change, that has not been the case for Sweden. It has a minority government led by the Social Democrats, whose formal position of military nonalignment was confirmed at their party congress in November.
However complicated NATO membership looks for Sweden politically, it would be dangerous to be left outside the alliance if Finland joins, since the two countries are each other’s closest defense partners and plan for war together, Ms. Wieslander, a Swede who is the Atlantic Council’s director for Northern Europe, said in an interview.
“We always consider Finnish security together with our own,” she said.
Opinion is shifting quickly in Sweden, too, with about 50 percent of people now in favor of joining NATO, rising to 62 percent if Finland joins, Ms. Wieslander said. In Finland, a recent poll had 68 percent in favor joining the alliance, rising to 77 percent if the president and government recommend it.
In Sweden, an all-party parliamentary group led by Foreign Minister Ann Linde is studying the issue, with a report due May 31. That deadline may be accelerated, because a decision to join NATO would have to pass Parliament with a solid majority, and that would depend on the Social Democrats shifting their position, Ms. Wieslander said.
In the last election of 2018, the Social Democrats’ vote share fell to 28.3 percent, their lowest since 1908. This makes them more sensitive to public opinion now than before, and just this week, the party announced that it was reconsidering its position on NATO.
Carl Bildt, a former Swedish prime minister, wrote recently in The Washington Post that after the war on Ukraine, “There is no way back to a past of illusionary neutrality.”
Given both countries’ relationships with NATO, applications to join the 30-member alliance would be accepted quickly, probably in late June, at NATO’s summit meeting in Madrid.
But all member states and their legislatures would have to ratify that decision, which took about a year for the last country to join, North Macedonia.
Russia-Ukraine War: Key Developments
In the meantime, Mr. Niinisto has discussed with President Biden and the British government the possibility of a bilateral or trilateral security guarantee.
Such a guarantee could be politically controversial, but might appeal to Britain. And Washington already has a trilateral defense cooperation agreement with Sweden and Finland that could be broadened.
For the Russians, Ms. Wieslander said, “you’re either under the shield or you’re not,” so security guarantees would have to be public and clear. Of course, she said, Sweden “is ready to shoot, if necessary — we don’t have to be formal allies for that.”
Russian responses are in any case likely to include internet disruptions, hacking of key ministries and disinformation efforts aimed especially at legislators who would have to vote on the issue. Tensions along the borders would increase, as would Russian fighter jets seeming to challenge airspace.
The United States has publicly supported the idea of membership. Julianne Smith, the American ambassador to NATO, said the United States would welcome both countries. “We’ve exercised; we’ve trained with them. They bring very capable militaries,” she said. “They are some of our closest allies in Europe, and so I can’t imagine a situation where there would be tremendous resistance to this idea.”
But it is Russia’s sudden war on Ukraine that has turned old assumptions hollow. The Ukrainian president, Volodymyr Zelensky, reminded Finns of the dangers Moscow presents in a speech on Friday to the Finnish Parliament, recalling the Winter War of 1939-40, when Stalin’s Soviet Union attacked Finland.
“You have seen Russia attack your country, and that threat still exists,” Mr. Zelensky said. “What they did in Bucha, they will do in your cities.” And it was Finnish courage then that has inspired Ukrainians in their own fight, he said.
Steven Erlanger reported from Brussels and Johanna Lemola from Helsinki.