That breakthrough came during a NATO summit in Madrid that has already become one of the most consequential meetings in the history of the military alliance.
The two countries are now expected to become full NATO members quickly, shoring up the bloc’s eastern flank within months of Russia’s invasion of Ukraine.
Here’s all you need to know about why the move happened, what comes next and why it matters.
What are the latest developments?
Sweden and Finland both announced their intention to join NATO in May, after Russia’s invasion of Ukraine caused a sudden shift in attitudes toward joining the bloc.
That announcement was welcomed by almost all of NATO’s leaders – but there was one significant obstacle. Turkey’s President, Recep Tayyip Erdogan, said he was not looking at both countries joining NATO “positively,” accusing them of housing Kurdish “terrorist organizations.”
Under NATO rules, just one member state can veto a new applicant’s membership.
However, a big diplomatic breakthrough between the three countries took place at the NATO summit in Madrid on Tuesday. Turkey signed a trilateral memorandum with Finland and Sweden, lifting its opposition and officially welcoming them to join the bloc.
“In NATO, we have always shown that whatever our differences, we can always sit down, find common ground and resolve any issues. NATO’s open door policy has been a historic success,” NATO Secretary General Jens Stoltenberg told journalists in Madrid.
On Wednesday, NATO formally invited Sweden and Finland to join, kickstarting a multistage process that will end with both countries holding full membership.
What happens next?
Stoltenberg said Wednesday that he expects Sweden and Finland to become members of the military alliance quickly.
The invitation sparks a seven-step accession process. Key moments along that path include talks between NATO and the candidate countries. The candidates must formally accept the obligations of membership, and then current member states sign an Accession Protocol, before individually ratifying it back home.
“We need a ratification process in 30 parliaments – that always takes some time but I also expect that to go rather quickly because allies are ready to try to make that ratification process happen as quickly as possible,” Stoltenberg explained Wednesday.
After that, the candidate country is formally invited to accede to the Washington Treaty, the founding document of the alliance.
NATO has an “open door” policy – any country can be invited to join if it expresses an interest, as long as it is able and willing to uphold the principles of the bloc’s founding treaty.
The ratification process usually takes about a year, from the signing of the Accession Protocol by existing members to the country joining the Washington Treaty.
But the war in Ukraine has added unprecedented urgency to Sweden and Finland’s membership, and the timeline could be accelerated accordingly.
How have leaders reacted?
US President Joe Biden praised the breakthrough with Turkey, saying it sent a clear signal to Russia that NATO was united and growing.
Sweden and Finland’s “decision to move away from neutrality and the tradition of neutrality to join the NATO alliance is going to make us stronger and more secure and NATO stronger,” Biden said. “We are sending an unmistakable message in my view … that NATO is strong, united, and the steps we are taking during this summit are going to further augment our collective strength.”
Biden said the two Nordic countries’ accession was a sign Putin’s aims had backfired.
“Putin was looking for the Finlandization of Europe,” he said, referring to the so-called dynamic Finnishization that saw Russia dominant over the foreign policy of its smaller neighbor for decades. “He’s going to get the NATOization of Europe, and that’s exactly what he didn’t want, that’s exactly what needs to be done to guarantee security for Europe. And I think it’s necessary,” Biden said.
The move was met with delight across the countries that make up NATO’s eastern front, many of which have expressed concern that they could be next in Russia’s crosshairs if it is successful in Ukraine.
Estonian Prime Minister Kaja Kallas said the step was “significant,” and Lithuanian President Gitanas Nauseda called it “wonderful news.”
What does NATO membership entail?
Article 5 has been a cornerstone of the alliance since it was founded in 1949 as a counterweight to the Soviet Union.
The point of the treaty, and Article 5 specifically, was to deter the Soviets from attacking liberal democracies that lacked military strength. Article 5 guarantees that the resources of the whole alliance – including the massive US military – can be used to protect any single member nation, such as smaller countries that would be defenseless without their allies. Iceland, for example, has no standing army.
Former Swedish leader Carl Bildt told CNN didn’t foresee new big military bases being built in either country if they joined. He said joining the alliance would probably mean more joint military training and planning between Finland, Sweden and the 30 current members. Swedish and Finnish forces could also participate in other NATO operations around the globe, such as those in the Baltic states, where several bases have multinational troops.
“There’s going to be preparations for contingencies as part of deterring any adventures that the Russians might be thinking of,” Bildt said. “The actual change is going to be fairly limited.”
Why haven’t Finland and Sweden already joined NATO?
While other Nordic countries like Norway, Denmark and Iceland were original members of the alliance, Sweden and Finland did not join the pact for historical and geopolitical reasons.
Both Finland, which declared independence from Russia in 1917 after the Bolshevik revolution, and Sweden adopted neutral foreign policy stances during the Cold War, refusing to align with either the Soviet Union or the United States.
Sweden opted to maintain its neutral status after the war ended.
Finland’s neutrality has historically proved more difficult, as it shared a long border with an authoritarian superpower.
A Finno-Soviet treaty known as the Agreement of Friendship, signed in 1948 and extended on occasion through the decades, prohibited Finland from joining any military alliance considered hostile to the USSR, or from allowing a Western attack through Finnish territory.
To keep the peace, Finns adopted an arrangement sometimes called Finlandization, in which leaders acceded to Soviet demands from time to time. The term was coined during the Cold War and has been applied to other countries in which a superpower exerts control over smaller neighboring states.
Both countries’ balancing acts effectively ended with the collapse of the Soviet Union. Sweden and Finland joined the European Union together in 1995 and gradually aligned their defense policies with the West, while still avoiding joining NATO outright.
How Russia’s invasion changed everything
Sweden and Finland have been inching toward the West on security issues since joining the EU shortly after the end of the Cold War. But Russia’s invasion of Ukraine dramatically accelerated that process, pushing them to pull the trigger on NATO membership.
If the Kremlin was willing to invade Ukraine – a country with 44 million people, a GDP of about $ 516 million, and armed forces of 200,000 active troops – what would stop Putin from invading smaller countries like Finland or Sweden?
“Everything changed when Russia invaded Ukraine,” Finnish Prime Minister Sanna Marin said in April. “People’s mindset in Finland, also in Sweden, changed and shifted very dramatically.”
Since the invasion of Ukraine in February, Finnish public support for joining NATO has leaped from around 30% to nearly 80% in some polls. The majority of Swedes also approve of their country joining the alliance, according to opinion polls there.
How has Russia reacted?
Russia lambasted the May decision by Finland and Sweden to seek to join the alliance. Its Deputy Foreign Minister, Sergei Ryabkov, said at the time that the move would be a “mistake” with “far-reaching consequences,” according to Russian state news agency TASS.
That followed similar threats from high-ranking Moscow officials. Kremlin spokesman Dmitry Peskov said after the announcement that “NATO expansion does not make the world more stable and secure.” He added that Russia’s reaction would depend on “how far and how close to our borders the military infrastructure will move.”
Russia currently shares about 755 miles of land border with five NATO members, according to the alliance. Finland’s accession would mean that a nation with which Russia shares an 830-mile border would become formally militarily aligned with the United States.
The addition of Finland and Sweden would also benefit the alliance, which would frustrate Russia. Both are serious military powers, despite their small populations.
But Putin has so far been more muted into his rhetoric than some of his officials. Last month he said that “Russia has no problems with these states,” adding that the expansion of NATO “does not pose a direct threat to Russia.”
“But the expansion of military infrastructure into this territory will certainly cause our response,” he added at the Collective Security Treaty Organization in Moscow. “We will see what it will be based on the threats that will be created for us.”
Why is Russia so opposed to NATO?
Putin sees the alliance as a defense against Russia, despite the fact that it spent much of the post-Soviet era focusing on issues like terrorism and peacekeeping.
Before Putin invaded Ukraine, he made clear his belief that NATO had edged too close to Russia and should be stripped back to its borders of the 1990s, before some countries that either neighboring Russia or were ex-Soviet states joined the military alliance.
Ironically, his invasion has given the alliance a new purpose – and increased its strength.
CNN’s Luke McGee, Nic Robertson, Paul LeBlanc, Per Bergfors Nyberg and Niamh Kennedy and Reuters contributed to this report.