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For many, that argument gives Russia’s president a pretext to prolong the war, which President Vladimir Putin ostensibly launched to keep Ukraine from joining NATO.

But if Ukraine joins NATO in the middle of the war, then Russia is at war with the entire NATO alliance.

At the core of this conundrum is Article 5 - NATO’s common defense guarantee under which an attack on one member is considered an attack on them all.

WHAT DOES ARTICLE 5 SAY?

Article 5 is at the heart of the 31-member North Atlantic Treaty Organization. It states that an armed attack against one or more of the members shall be considered an attack against all members.

It also states that if such an armed attack occurs, each of the members would take, individually and in concert with other members, ‘’such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.’’

That security guarantee is the reason previously neutral Finland and Sweden sought to join NATO and why Ukraine and other countries in Europe also want in.

WHEN HAS IT BEEN INVOKED?

Only once, in the wake of the September 11, 2001 terror attacks on the United States, paving the way for NATO’s biggest ever operation in Afghanistan.

But NATO allies have also taken collective defense measures including joining the U.S. to fight the Islamic State group in Syria, Iraq and Afghanistan as well as help keep the peace in the Balkans.

HOW DOES THIS IMPACT UKRAINE?

NATO’s credibility hinges on Article 5 and its commitment to offer membership to any European country that can contribute to security in Europe and North America.

But Ukraine, currently in the middle of war with Russia, would oblige all 31 member countries to spring to its defense militarily, potentially igniting a wider war with a nuclear-armed country.

Article 5 also becomes problematic when the territory of a member is unclear. For instance, Russian forces entered Georgia in August 2008, a few months after NATO leaders first promised the country it would join, along with Ukraine. That country's NATO application is still pending. Russia continues to occupy large swaths of Ukraine in the east and south, further muddying the picture.

U.S. President Joe Biden summed up why he didn't think Ukraine is ready for NATO membership. ‘’If the war is going on, then we're all in the war... we're at war with Russia, if that were the case,'' he said in an interview with CNN ahead of the summit.