There are 30 countries that are currently part of NATO.
Aspiring nations have to meet certain political, economic, and military standards to join.
Ukraine's pursuit of NATO membership has been cited as a key factor in Russia's invasion.
Two people were killed after Russian missiles reportedly landed in a Polish village on Tuesday, a US intelligence officer told the Associated Press, prompting immediate questions of how the North Atlantic Treaty Organization would respond to an apparent attack on a member country.
NATO has played a key role in Russia's nearly nine-month assault in Ukraine, and President Vladimir Putin's unprovoked war has even pushed certain longtime non-member nations to apply to join the alliance.
Ukraine's pursuit of NATO membership — a quest intrinsically aligned with Western expansionism — has been cited as a key factor in Putin's decision to invade the former Soviet territory in February. Ukrainian President Volodymyr Zelensky has long been vocal about his country's desire to secure both NATO membership and additional assistance.
Russia denied reports that its weapons landed in Poland. US officials have not confirmed the reports.
What is NATO?
The North Atlantic Treaty Organization is a military alliance created in 1949 to provide collective security against Soviet expansionism and to encourage European political integration in the aftermath of World War II.
The alliance contained just 12 countries when it was founded but has more than doubled in size in the decades since. The body now consists of two countries in North America and 28 European countries, including several former Soviet nations.
What does it take to become a member of the alliance?
NATO employs an "open door policy" for aspiring members.
"Any European country in a position to further the principles of the Washington Treaty and contribute to security in the Euro-Atlantic area can become a member of the Alliance at the invitation of the North Atlantic Council," according to NATO's website.
Thus, any European country that independently decides to pursue NATO membership may do so. But nations that wish to join must meet certain political, economic, and military standards.
While there is no official checklist for membership, the alliance maintains a list of minimum requirements that aspiring countries should be able to meet:
New members must uphold democracy, including tolerating diversity
New members must be making progress toward a market economy
Their military forces must be under firm civilian control
They must be good neighbors and respect sovereignty outside their borders
They must be working toward compatibility with NATO forces
Once a country makes its desire to join the alliance known, NATO may invite the country to join the Membership Action Plan, which is a program that helps nations prepare for future membership, though participation does not guarantee membership, according to NATO's website.
Why isn't Ukraine a member of NATO already?
Ukraine has expressed a desire to join NATO but has never been formally admitted.
The alliance has, however, designated the former Soviet country as one of its "enhanced opportunity partners," a title granted to non-member countries that have contributed to NATO-led operations and missions.
In 2008, the country applied to begin a NATO Membership Action Plan and the alliance welcomed Ukraine's bid, pledging that the country would eventually become a member, though declining to offer a specific timeline.
When former Ukrainian President Viktor Yanukovych was elected in 2010, plans to move forward with NATO membership were scrapped due to Yanukovych's desire to remain non-aligned. But the leader fled the country in February 2014 amid Russian aggression and national unrest.
Since that time, Ukraine and its leaders have continued to make NATO membership a priority and public support for Ukraine's inclusion has steadily grown over the years.
But ongoing unrest in parts of Ukraine, even before 2022, worried some NATO members — such as France and Germany, two countries that previously opposed Ukraine's inclusion — and kept official membership out of Ukraine's reach, despite the 2008 promise.
"The feeling was, and probably still has been, that Ukraine hadn't completely taken care of political corruption, that it was still developing its democracy," Stanley Sloan, an expert in transatlantic relations at Middlebury College and a former international security officer, told McClatchy News. "So there were some formal reasons why the Alliance could say that Ukraine was not ready yet to join the Alliance."
Politics played a role, as well. For years, Putin sought assurances that Ukraine would never join the organization, highlighting his concern that the alliance's eastward expansion represents a direct threat to Russia. As such, some NATO members have remained staunchly against Ukraine's inclusion because of the real or perceived consequences Russia threatened as a result.
And a new country can be inducted only if the full alliance, which employs consensus decision-making, is in agreement.
Will Finland and Sweden join NATO?
Both countries submitted official letters of application to join NATO in May 2022 and were formally invited to join the alliance in June.
All 30 current member countries must now vote on their inclusion. As of November, 28 countries had voted in favor of including Sweden and Finland, with Hungary and Turkey the only two countries left to vote.
Both countries have a history with the alliance as members of its Partnership for Peace program, which allows partners to "build up an individual relationship with NATO, choosing their own priorities for cooperation."
But since World War II, Finland and Sweden have both remained militarily neutral, a stance that previously kept them out of NATO by choice. Finland, which shares an 830-mile border with Russia, long maintained a security policy "designed to withstand times of crisis," the country's foreign minister Pekka Haavisto said in February.
But escalating Russian aggression seemingly pushed the country toward NATO membership, much to Putin's chagrin. Russian officials in February warned of "serious military-political consequences" from Moscow should Finland or Sweden join the alliance.
Sweden has also been a longtime example of European neutrality, relying on trade with both the West and Russia for decades. But in recent years, Russian aggression close to home has forced Sweden to pad its military capabilities and as a result, public support for NATO inclusion slowly began to creep upwards, according to iNews.
Leaders from both countries have brushed off Russia's warnings, with Finnish Foreign Minister Pekka Haavisto saying "we've heard this before." Finish President Sauli Niinisto said Moscow's comments were not a military threat to Finland, but instead, hint at the sort of "countersteps" Russia would likely take if Finland joined the alliance.
Top officials from both Sweden and Finland have emphasized the importance of individual choice when it comes to NATO inclusion.
"It is very important that NATO keeps its open-door policy; that Finland keeps the right to apply, and that is our position for Ukraine and Georgia as well," Haavisto said. "Every country should have that right."
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