That breakthrough came during a NATO summit in Madrid that has already become one of the most follow-up meetings in the history of the military alliance.
The two countries are now expected to become full NATO members quickly, supporting the bloc’s eastern flank within months of Russia’s invasion of Ukraine.
Here is everything you need to know about why the move happened, what comes next and why it is important.
What is the latest development?
Sweden and Finland both announced their intention to join NATO in May, after Russia’s invasion of Ukraine caused a sudden change in attitudes towards joining the bloc.
That announcement was welcomed by almost all NATO leaders – but there was a significant obstacle. Turkish President Recep Tayyip Erdogan said he did not view both countries’ accession to NATO as “positive” and accused them of hosting Kurdish “terrorist organizations”.
According to NATO rules, only one member state can veto a new applicant’s membership.
However, a major 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, raised its opposition and officially welcomed them to join the bloc.
“In NATO, we have always shown that regardless of our differences, we can always sit down, find a common ground and solve all problems. NATO’s open door policy has been a historic success,” NATO Secretary General Jens Stoltenberg told reporters in Madrid.
On Wednesday, NATO formally invited Sweden and Finland to join, which kick-started a multi-step process that will end with both countries holding full membership.
What happens next?
Stoltenberg said on Wednesday that he expects Sweden and Finland to quickly become members of the military alliance.
The invitation initiates a connection process in seven steps. Important moments along that path include talks between NATO and the candidate countries. Candidates must formally accept the obligations of membership, and then the current member states sign an accession protocol, before they individually ratify it at home.
“We need a ratification process in 30 parliaments – it always takes some time but I also expect it to go pretty fast because allies are ready to try to get that ratification process done as quickly as possible,” Stoltenberg explained on Wednesday.
The candidate country is then formally invited to accede to the Washington Treaty, the alliance’s basic document.
NATO has an “open door” policy – any country can be invited to join if it expresses an interest, as long as it can and wants to uphold the principles of the bloc’s founding treaty.
The ratification process usually takes about a year, from the time the existing members sign the accession protocol until the country accedes to the Washington Treaty.
But the war in Ukraine has made Sweden and Finland’s membership even more urgent, and the timeline could be accelerated accordingly.
How have the leaders reacted?
US President Joe Biden praised the breakthrough with Turkey and said it was sending a clear signal to Russia that NATO is united and growing.
Sweden and Finland’s “decision to move away from neutrality and the tradition of neutrality to join the NATO alliance will make us stronger and more secure and NATO stronger”, said Biden. “We are sending an unmistakable message in my opinion … that NATO is strong, united, and the steps we are taking during this summit will further increase our collective strength.”
Biden said that the accession of the two Nordic countries was a sign that Putin’s goal had backfired.
“Putin was looking for the Finnishisation of Europe,” he said, referring to the so-called Finnishisation dynamics that saw Russia dominate over its smaller neighbor’s foreign policy for decades. “He will bring about a NATO isolation of Europe, and that is exactly what he did not want, that is exactly what needs to be done to ensure security for Europe. And I think that is necessary,” Biden said.
The move was met with joy over the countries that make up NATO’s eastern front, many of which have expressed concern that they could be next in Russia’s cross if successful in Ukraine.
Estonian Prime Minister Kaja Kallas said the move was “significant”, and Lithuanian President Gitanas Nausėda called it “wonderful news”.
What does NATO membership mean?
Article 5 has been a cornerstone of the alliance since its founding in 1949 as a counterweight to the Soviet Union.
The purpose 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 entire 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.
The former Swedish leader Carl Bildt told CNN that they did not anticipate that new large military bases would be built in any of the countries if they joined. He said that 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 take part in other NATO operations around the world, for example in the Baltic states, where several bases have multinational troops.
“There will be preparations for unforeseen events as part of discouraging any adventures that the Russians might think of,” Bildt said. “The actual change will be quite limited.”
Why have not Finland and Sweden already joined NATO?
While other Nordic countries such as 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 took neutral foreign policy positions during the Cold War and refused to join either the Soviet Union or the United States.
Sweden chose to maintain its neutral status after the end of the war.
Finland’s neutrality has historically proved more difficult, as it shared a long border with an authoritarian superpower.
A Finnish-Soviet treaty known as the Friendship Agreement, signed in 1948 and sometimes extended for decades, forbade Finland to join any military alliance that was considered hostile to the Soviet Union, or to allow a Western attack through Finnish territory.
In order to maintain peace, the Finns adopted an arrangement sometimes called Finnishization, in which the leaders occasionally agreed to Soviet demands. The term was coined during the Cold War and has been applied to other countries where a superpower exercises control over smaller neighboring states.
The balancing acts of both countries ended in practice with the collapse of the Soviet Union. Sweden and Finland joined the European Union together in 1995 and gradually adapted their defense policy to the Western world, while avoiding joining NATO directly.
How Russia’s invasion changed everything
Sweden and Finland have been heading west in security matters since joining the EU shortly after the end of the Cold War. But Russia’s invasion of Ukraine dramatically accelerated that process, forcing it to pull the trigger on NATO membership.
If the Kremlin were willing to invade Ukraine – a country of 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,” said Finnish Prime Minister Sanna Marin in April. “The way of thinking of people in Finland, including Sweden, changed and changed very dramatically.”
Since the invasion of Ukraine in February, the Finnish public’s support for joining NATO has increased from around 30% to almost 80% in some opinion polls. The majority of Swedes also approve of their country joining the alliance, according to opinion polls there.
How has Russia reacted?
Russia criticized Finland’s and Sweden’s decision in May to try to join the alliance. Its deputy foreign minister, Sergei Ryabkov, said the move was a “mistake” with “far-reaching consequences”, according to the Russian state news agency TASS.
It 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 response would depend on “how far and 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 km long border would be formally militarily connected to 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 population.
But Putin has so far been more subdued in his rhetoric than some of his officials. Last month, he said that “Russia has no problems with these states”, adding that NATO’s expansion “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 happens 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 spending much of the post-Soviet era focusing on issues such as terrorism and peacekeeping.
Before Putin invaded Ukraine, he clarified his conviction that NATO had come too close to Russia and should be withdrawn to its borders in the 1990s, before any countries either bordering Russia or 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.