Polish Prime Minister Donald Tusk said on Wednesday he had asked Nato to open consultations under Article 4 of the alliance's treaty after Poland shot down drones in its airspace following what he called a "large-scale provocation" by Russia.
Poland said it had downed drones over its territory during a large Russian air attack on Ukraine, the first time a Nato member state is known to have fired shots in the war.
Russia's defence ministry said its drones had carried out an attack on military facilities in western Ukraine but that it had not planned to hit any targets in Poland. The Russian drones "which allegedly crossed the border with Poland" had a range of not more than 700 km, it said.
Several European officials described the incursion as intentional and a sign of Russian escalation. Tusk said it was "the closest we have been to open conflict since World War Two" but that he had "no reason to believe we're on the brink of war".
Article 4 states that members of the North Atlantic Treaty Organisation will consult together whenever, in the opinion of any of them, the territory, political independence or security of any of them is threatened.
Under Article 4, discussions at the North Atlantic Council - Nato’s principal political decision-making body - could potentially lead to some form of joint decision or action.
Since Nato’s creation in 1949, Article 4 has been invoked seven times, most recently in February 2022 when Bulgaria, the Czech Republic, Estonia, Latvia, Lithuania, Poland, Romania and Slovakia sought consultations following Russia's war on Ukraine.
Nato ambassadors in November 2022 held an emergency meeting after a missile strike killed two people in Poland and raised global alarm that the war could spill into neighbouring countries.
If Russia were determined to have attacked the territory of a member state, the focus would then shift to Article 5, the cornerstone of the founding treaty of Nato.
The alliance was created in 1949 with the US 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-defence recognised 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.
Since Ukraine is not part of Nato, Russia's full-scale war in February 2022 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 neighbouring 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.
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 war against another.
Article 5 has been activated once before - on behalf of the United States, in response to the September 11, 2001, hijacked-plane attacks on New York and Washington.
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