Why NATO formed, why Finland joined and why Sweden wants to join

• It's NATO's ninth enlargement since its founding in 1949.
• Finland, along with Sweden, applied for NATO membership "hand in hand" after Russia invaded Ukraine last year.
• The two Nordic neighbors had hoped to become members at the same time, but Hungary and Turkey have stalled Sweden's application. It's unclear if and when it may move forward.

Below is a brief look at why NATO was formed, why Finland and Sweden applied for membership, and where the alliance's expansion leaves Russia and Ukraine.

Why was NATO formed?

• The North Atlantic Treaty Organization, or NATO, was created in 1949 by 12 countries, including the U.S., Canada and other Western European countries, to provide collective security against the Soviet Union.
• Since the fall of the Soviet Union, NATO has more than doubled in size to include 28 European countries, Canada and the U.S. It added its 31st member, Finland, on April 4, 2023.
• Today, NATO's stated purpose is "to guarantee the freedom and security of its members through political and military means."

When does NATO act militarily?

• Article 5 — which stands at the heart of NATO's founding treaty — says that an attack on any member of the alliance should be viewed as an attack on all members.

• If such an attack does occur, each member will take "measures necessary to restore and maintain international peace and security," according to the treaty.
• Article 5 has been invoked just one time in history — after the terrorist attacks on Sept. 11, 2001, when the alliance launched aircraft to help patrol the skies over the U.S.

How do countries join NATO?

• A country seeking to join the mutual-defense alliance must demonstrate that they meet political, economic and military goals and that it will both contribute to and benefit from NATO's collective security.
• Requirements for entry include having a functioning democratic political system based on a market economy, fair treatment of minority populations, and a commitment to democratic civil-military relations, according to the alliance.
• All NATO member states must support a country's membership.

Why do Finland and Sweden want to join?

Finland and Sweden have for decades been NATO's closest partners, despite their official "non-alignment."

• Public support for NATO membership in the Nordic countries shot up virtually overnight after Russia invaded Ukraine, with a strong majority in both countries favoring joining the alliance, Axios' Zachary Basu writes.
• The two countries jointly submitted their applications for membership last May.

Finland's President Sauli Niinistö at the time said his country sought NATO membership because Russia's invasion proved that the Kremlin does not respect officially non-aligned countries.

• "What we see now, Europe, the world, is more divided," Niinistö told CNN. "There's not very much room for nonaligned in between."

Sweden's Prime Minister Magdalena Andersson also pointed to Russia's actions.

• "Russia's unprovoked invasion of Ukraine is not only illegal and indefensible, it also undermines the European security order that Sweden builds its security on," Andersson said in a speech at the time.
• "Should Sweden be the only country in the Baltic Sea region that was not a member of NATO, we would be in a very vulnerable position. We can't rule out that Russia would then increase pressure on Sweden," she warned.

NATO Secretary-General Jens Stoltenberg in welcoming Finland to the alliance in April said: "President Putin wanted to slam NATO's door shut. Today we showed the world that he failed, that aggression and intimidation do not work. Instead of less NATO, he has achieved the opposite: more NATO. And our door remains firmly open."

• Stoltenberg added he looked forward to welcoming Sweden "as soon as possible."

Ukraine and NATO

Ukraine, along with Georgia, sought to join NATO at the Bucharest Summit in 2008, viewing it as a route to independence from Russian influence, per the Washington Post.

• The U.S. and several other NATO members supported the countries' membership, but Germany and France blocked the effort, arguing that Ukraine membership would outrage Russia. They also contended the two countries were not ready for membership.

• In an attempted compromise, Germany and France said the two countries could become members — but did not specify when.
• In the months that followed, Ukraine and Georgia's vague assurances of membership dwindled. Talks between Ukraine and NATO resumed at the end of 2008, but no specific Membership Action Plan (MAP) was announced, per the Post.

Ukraine's hopes of joining NATO were then dashed in 2010 when Viktor Yanukovych was elected president. After taking office, Yanukovych called joining the alliance an "unrealistic prospect."

• Since Yanukovych was ousted from power in 2014, Ukraine has renewed its interest in joining NATO, including in 2017, when it adopted legislation making NATO membership a priority, but it hasn't advanced its bid for membership.
• Ukrainian President Volodymyr Zelensky has accepted that his country will not join NATO, at least in the current moment. "It is clear that Ukraine is not a member of NATO. We understand this," he said in March 2022.

Russia and NATO

• Putin is staunchly opposed to Ukrainian membership in NATO, arguing that eastward expansion of the alliance would pose a threat to Russia's security.
• The Russian president last year said that NATO "in essence, is engaged in a war with Russia through a proxy and is arming that proxy."
• Putin had warned Finland and Sweden that NATO membership would be a "mistake." He later said that "there is no direct threat to Russia in connection with NATO's expansion to these countries. But the expansion of [the alliance's] military infrastructure to these territories will certainly evoke a response on our part."
• The Kremlin later called Finland's NATO accession an “encroachment on Russia’s security” that would require Russia to take unspecified countermeasures.

NATO forces and Ukraine

• NATO is taking collective defense measures in response to Russia's invasion, without invoking Article 5.
• Since Russia's military buildup on Ukraine's border began last October, the U.S. and NATO have deployed thousands of troops to eastern-flank countries like Poland, Lithuania, Latvia, Estonia, Bulgaria, and Romania.
• NATO has also sent thousands of weapons to Ukraine throughout the invasion, including antitank weapons, artillery systems and helicopters.
• Early in the war, NATO ruled out establishing a "no-fly zone" over Ukraine, despite repeated requests from Zelensky to establish one, fearing that it could mark a significant escalation and bring the alliance directly into a conventional conflict with a nuclear power.

Go deeper:

• Finland and Sweden formally apply to join NATO
• Where 100,000 U.S. troops are stationed in Europe
• Why Ukraine wants a no-fly zone — but is unlikely to get one

Editor's note: This story has been updated with new details throughout.