Why isn't Ukraine already in NATO? Here's what it takes to join the 30-country alliance

The North Atlantic Treaty Organization is having a moment in the spotlight as Russia's ongoing assault on Ukraine pushes key non-member nations to consider joining the alliance.

The decision to invade the former Soviet territory by Russian President Vladimir Putin was linked to the pursuit of NATO membership by Ukraine.

In the days since Russia launched its attack, Ukrainian President Volodymyr Zelensky has been vocal about his country's desire to join NATO.

Who is ready to defend us? Zelensky said that he didn't see anyone who was ready to give Ukraine a guarantee of joining NATO. Everyone is afraid.

Pro-NATO sentiment is spreading to other non-members as the conflict rapidly escalates.

The NATO summit last week saw a new interest in joining the alliance from two of the most neutral countries on the planet, Sweden and Finland.

What is NATO?

The North Atlantic Treaty Organization was formed in 1949 to provide collective security against Soviet expansionism and to encourage European political integration after World War II.

NATO serves as a collective security system, whereby its member states agree to mutually defend any attack on a member party.

When the alliance was founded, it contained just 12 countries. The body now has two countries in North America and 28 European countries.

What does it take to become a member of the alliance?

NATO has 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.

Any European country that decides to pursue NATO membership can do so. Nations that wish to join must meet certain standards.

The alliance has 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

NATO may invite a 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 the NATO website.

AP Photo/Olivier Matthys

Why isn't Ukraine a member of NATO already?

NATO has never formally admitted Ukraine, despite the fact that it has expressed a desire to join.

The former Soviet country has been designated an enhanced opportunity partner by the alliance.

In 2008 the country applied to begin a NATO Membership Action Plan and the alliance welcomed the country, though it did not give a specific time frame.

Plans to join NATO were scrapped due to the desire of the former Ukrainian President to remain non-aligned. The leader fled the country in February of last year.

Over the years, NATO membership has been a priority for the leaders of Ukraine and they have continued to make it a priority.

The unrest in parts of Ukraine worried some NATO members, such as France and Germany, which had previously opposed the inclusion of the country in the alliance.

Stanley Sloan, an expert in transatlantic relations at Middlebury College and a former international security officer, said that the feeling was that Ukraine hadn't taken care of political corruption.

Politics played a role.

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. Some NATO members have remained steadfast against the inclusion of Ukraine because of the real or perceived consequences that Russia could have had.

If the full alliance is in agreement, a new country can be formed.

Could Finland and Sweden join NATO?

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.

Since World War II, both Sweden and Finland have remained neutral, keeping them out of NATO. Despite the attack by Russia, the leaders in Sweden and Finland decided to stay out of the alliance.

Last week, the country's foreign minister Pekka Haavisto said that the security policy was designed to endure times of crisis.

The country is closer to NATO membership than ever before because of the Russian aggression.

There is a spike in support from the Finns. A poll found that only a small percentage of Finns supported joining the alliance. If Sweden joined the alliance, the percentage of Finns who support it would jump to 34%, according to a new survey.

Sweden has long been a key example of European neutrality, relying on trade with both the West and Russia for decades.

Following an emergency NATO summit on Friday morning, the alliance extended an invitation to both Sweden and Finland to be part of an intensified exchange of information regarding NATO's strategic communications.

The arrangement does not necessarily mean NATO membership is on the horizon, but it was enough for Putin to be angry. Russian officials this weekend warned of serious military-political consequences from Moscow if Sweden joined the alliance.

Russian Foreign Ministry spokeswoman Maria Zakharova said in a statement that the countries may be interested in entering the alliance because of the US's attempts to get them into NATO.

The leaders of both countries brushed off Russia's warning, with Finish President Sauli Niinisto saying that Moscow's comments were not a military one.

The importance of individual choice has been emphasized by top officials from both Sweden and Finland.

Haavisto said that NATO keeps its open-door policy, that Finland keeps the right to apply, and that Georgia and Ukraine should also have that right.