This breakthrough came during the NATO summit in Madrid, which has already become one of the most important meetings in the history of the military alliance.
The two countries are now expected to quickly become full NATO members, bolstering the bloc’s eastern flank in the months following Russia’s invasion of Ukraine.
Here’s everything you need to know about why the move happened, what’s next, and why it’s important.
What are the latest developments?
Sweden and Finland announced their intention to join NATO in May, after Russia’s invasion of Ukraine caused a sudden shift in attitudes towards joining the bloc.
This statement was welcomed by almost all NATO leaders, but there was one significant obstacle. Turkish President Recep Tayyip Erdogan said he did not look at the entry of both countries into NATO “positively”, accusing them of hosting Kurdish “terrorist organizations”.
Under NATO rules, only one member state can veto the membership of a new candidate.
However, there was a big diplomatic breakthrough between the three countries at the NATO summit in Madrid on Tuesday. Turkey has signed a tripartite memorandum with Finland and Sweden, lifting its resistance and officially welcoming them to join the bloc.
“At NATO, we have always demonstrated 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 reporters in Madrid.
On Wednesday, NATO formally invited Sweden and Finland to join, beginning a multi-stage process that will culminate in both countries becoming full members.
What happens next?
Stoltenberg said Wednesday he expects Sweden and Finland to quickly become members of a military alliance.
The invitation starts a seven-step entry process. Key milestones along the way include negotiations between NATO and candidate countries. Applicants must formally commit to membership, and current member states then sign the Protocol of Accession before individually ratifying it at home.
“We need a ratification process in 30 parliaments – it always takes some time, but I also expect it to happen quite quickly because the allies are ready to try to speed up the ratification process,” Stoltenberg explained on Wednesday.
After that, the candidate country is officially invited to join 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, provided it is able and willing to abide by 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’s accession to the Washington Treaty.
But the war in Ukraine has added an unprecedented urgency to the membership of Sweden and Finland, and the timeline could be accelerated accordingly.
How did the leaders react?
US President Joe Biden praised the breakthrough with Turkey, saying he sent a clear message to Russia that NATO is united and growing.
“The decision of Sweden and Finland to abandon neutrality and the tradition of neutrality and join the NATO alliance will make us stronger and safer, and NATO stronger,” Biden said. “In my opinion, we are sending an unmistakable signal … that NATO is strong, united, and the steps we are taking during this summit will further increase our collective strength.”
Biden said the accession of the two Scandinavian countries was a sign that Putin’s goals were not working.
“Putin was aiming for the Finnishization of Europe,” he said, referring to the so-called Finnishization dynamic, when Russia dominated the foreign policy of its smaller neighbor for decades. “He is going to achieve the Natonization of Europe, and this is exactly what he did not want, this is exactly what needs to be done to guarantee the security of Europe. And I think it’s necessary,” Biden said.
The move was greeted with enthusiasm in the countries that make up NATO’s eastern front, many of which expressed concern that they could be next in the crosshairs of Russia if it succeeds in Ukraine.
Estonian Prime Minister Kaja Kallas called the move “significant”, while Lithuanian President Gitanas Nauseda called it “great news”.
What does NATO membership entail?
Article 5 has been the cornerstone of the alliance since its founding in 1949 as a counterweight to the Soviet Union.
The point of the treaty, and especially Article 5, was to keep the Soviets from attacking liberal democracies that lacked military power. Article 5 ensures that the resources of the entire alliance, including the vast US military, can be used to protect any individual member state, such as small countries that would be defenseless without their allies. Iceland, for example, does not have a standing army.
Former Swedish leader Carl Bildt told CNN he does not foresee the construction of large new military bases in any of the countries if they join. He said joining the alliance would likely mean more joint military training and planning between Finland, Sweden and the 30 current members. Swedish and Finnish forces may also participate in other NATO operations around the world, such as in the Baltic States, where multinational troops are stationed at several bases.
“There will be contingency preparations as part of containing any escapades the Russians might think of,” Bildt said. “The actual change will be quite limited.”
Why haven’t Finland and Sweden joined NATO yet?
While other Scandinavian countries such as Norway, Denmark and Iceland were the original members of the alliance, Sweden and Finland did not join the pact due to historical and geopolitical reasons.
Both Finland, which declared independence from Russia in 1917 after the Bolshevik Revolution, and Sweden adopted a neutral foreign policy stance during the Cold War, refusing to join either the Soviet Union or the United States.
Sweden chose to maintain its neutral status after the end of the war.
Historically, Finland’s neutrality has proven more difficult because it has a long border with an authoritarian superpower.
The Soviet-Finnish treaty, known as the Friendship Agreement, signed in 1948 and extended from time to time for decades, forbade Finland from entering into any military alliance that was considered hostile to the USSR, or allowing the West to attack through Finnish territory.
To keep the peace, the Finns adopted an arrangement, sometimes called finlandization, in which the leaders agreed to Soviet demands from time to time. The term was coined during the Cold War and applied to other countries where a superpower exercises control over smaller neighboring states.
The balancing act of both countries actually ended with the collapse of the Soviet Union. Sweden and Finland joined the European Union together in 1995 and have gradually aligned their defense policies with the West, while avoiding direct entry into NATO.
How the Russian invasion changed everything
Sweden and Finland have been gradually moving closer to 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 this process, forcing them to pull the trigger for NATO membership.
If the Kremlin were ready to invade Ukraine, a country of 44 million people with a GDP of about $155 billion and a military force of 200,000 – what’s to stop Putin from invading smaller countries like Finland or Sweden?
“Everything changed when Russia invaded Ukraine,” Finnish Prime Minister Sanna Marin said in April. “The mindset of people in Finland and also in Sweden has changed very dramatically.”
After the February invasion of Ukraine, according to some polls, Finnish public support for joining NATO jumped from 30% to almost 80%. According to public opinion polls, the majority of Swedes also approve of their country’s entry into the alliance.
How did Russia react?
Russia criticized the May decision of Finland and Sweden to try to join the alliance. According to the Russian state news agency TASS, Russian Deputy Foreign Minister Sergei Ryabkov said at the time that the move would be a “mistake” with “far-reaching consequences.”
This followed similar threats from senior Moscow officials. Kremlin spokesman Dmitry Peskov said after the statement that “NATO expansion does not make the world more stable and secure.” He added that Russia’s reaction will depend on “how far and how close to our borders the military infrastructure will move.”
According to the alliance, Russia currently shares about 755 miles of land border with five NATO members. Finland’s accession would mean that the country with which Russia shares an 830-mile border would become a formal military ally of the United States.
Adding Finland and Sweden would also benefit the alliance, which would upset Russia. Both are serious military forces despite their small populations.
But so far, Putin has been more restrained in his rhetoric than some of his officials. Last month, he said “Russia has no problem with these states,” adding that NATO expansion “poses no direct threat to Russia.”
“But the expansion of military infrastructure into this territory will definitely provoke our reaction,” he added at the CSTO in Moscow. “Let’s see what it will be based on the threats that will be created for us.”
Why is Russia so anti-NATO?
Putin sees the alliance as a defense against Russia, despite his involvement in issues such as terrorism and peacekeeping for most of the post-Soviet era.
Before Putin invaded Ukraine, he made it clear that he believed that NATO had gotten too close to Russia and should be pulled back to its 1990s borders before some countries that either neighbored Russia or were former Soviet states joined the military alliance.
Ironically, his invasion gave the alliance a new purpose and increased its strength.
Correction: This story has been updated to correct for Ukraine’s GDP, which was $155.5 billion in 2020 according to the World Bank.
CNN’s Luke McGee, Nick Robertson, Paul LeBlanc, Per Bergfors Nyberg and Niamh Kennedy and Reuters contributed to this report.