EXPLAINER-NATO's Articles 4 and 5: How the Ukraine conflict could trigger its defense obligations

Since Ukraine is not part of NATO, Russia's invasion in February did not trigger Article 5, though the United States and other member states rushed to provide military and diplomatic assistance to Kyiv. However, experts have long warned of the potential for a spillover to neighboring countries on NATO's eastern flank that could force the alliance to respond militarily.

A deadly explosion occurred in NATO member Poland's territory near its border with Ukraine on Tuesday, and the United States and its allies said they were investigating unconfirmed reports the blast had been caused by stray Russian missiles. The explosion, which firefighters said killed two people, raised concerns of Russia's war in Ukraine becoming a wider conflict. Polish authorities said it was caused by a Russian-made rocket, but Russia's defense ministry denied involvement.

If it is determined that Moscow was to blame for the blast, it could trigger NATO's principle of collective defense known as Article 5, in which an attack on one of the Western alliance's members is deemed an attack on all, starting deliberations on a potential military response. As a possible prelude to such a decision, however, Poland has first requested a NATO meeting on Wednesday under the treaty's Article 4, European diplomats said. That is a call for consultations among the allies in the face of a security threat, allowing for more time to determine what steps to take.

The following is an explanation of Article 5 and what might occur if it is activated: WHAT IS ARTICLE 5?

Article 5 is the cornerstone of the founding treaty of NATO, which was created in 1949 with the U.S. military as its powerful mainstay essentially to counter the Soviet Union and its Eastern bloc satellites during the Cold War. The charter stipulates that "the Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all."

"They agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defense recognized by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area," it says. AND WHAT IS ARTICLE 4?

Article 4 states that NATO members "will consult together whenever, in the opinion of any of them, the territorial integrity, political independence or security of any of the Parties is threatened." Within hours of the blast in Poland on Tuesday, two European diplomats said that Poland requested a NATO meeting under Article 4 for consultations.

HOW COULD THE UKRAINE WAR TRIGGER ARTICLE 5? Since Ukraine is not part of NATO, Russia's invasion in February did not trigger Article 5, though the United States and other member states rushed to provide military and diplomatic assistance to Kyiv.

However, experts have long warned of the potential for a spillover to neighboring countries on NATO's eastern flank that could force the alliance to respond militarily. Such action by Russia, either intentional or accidental, has raised the risk of widening the war by drawing other countries directly into the conflict.

IS INVOKING ARTICLE 5 AUTOMATIC? No. Following an attack on a member state, the others come together to determine whether they agree to regard it as an Article 5 situation.

There is no time limit on how long such consultations could take, and experts say the language is flexible enough to allow each member to decide how far to go in responding to armed aggression against another. HAS ARTICLE 5 BEEN INVOKED BEFORE?

Yes. Article 5 has been activated once before – on behalf of the United States, in response to the Sept. 11, 2001, hijacked-plane attacks on New York and Washington. WHAT HAS BIDEN SAID ABOUT ARTICLE 5 COMMITMENTS?

While insisting that the United States has no interest in going to war against Russia, President Joe Biden has said from the start of Moscow's invasion that Washington would meet its Article 5 commitments to defend NATO partners. "America's fully prepared with our NATO allies to defend every single inch of NATO territory. Every single inch," Biden said at the White House in September.

He had declared earlier that there was "no doubt" that his administration would uphold Article 5.

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