NATO leaders have met after a missile struck eastern Poland, one of the alliance's member countries.
Investigators say the missile that exploded in its territory likely came from Ukraine, not Russia, in an incident which left many wondering how the Western allies will respond.
Here's a quick guide to what happened, why it matters and what could happen next.
What happened?
A missile stuck in Przewodow, a village in eastern Poland, near the Ukrainian border.
Two people died in the blast.
A statement from the Polish Foreign Ministry said the missile was Russian-made, however, it is believed it was probably a Ukrainian air defence missile.
There is no evidence to suggest the incident was an intentional attack by Russia.
However Russia is still copping some of the blame for starting the war against Ukraine.
US President Joe Biden said it was "unlikely" the missile was fired from Russia, but called an emergency meeting of the Group of Seven (G7) and NATO leaders who happened to be in Bali for the G20 summit.
Three anonymous US officials later told the Associated Press that an early assessment suggested the missile was fired by Ukrainian forces in an attempt to take out an incoming Russian missile.
Why is this important?
Because Poland is a member of NATO, and the terms of the treaty dictate that an attack on one NATO member country is considered an attack against all of them.
Does this mean NATO countries will immediately launch a counter-attack?
No.
Polish authorities are still investigating the attack and working out where the missile came from.
Foreign Affairs Minister Penny Wong said the NATO allies were "demonstrating the sort of response we would want".
"There is calm, consultation [and] a lot of engagement," she said.
What happens now?
It was possible for Poland to request consultations with NATO countries under Article 4 of the treaty, however Polish Prime Minister Prime Minister Mateusz Morawiecki said Warsaw might not need to.
The requested depended on the investigation into the missile's origin.
"NATO and Poland should take their time to investigate, to determine who's the culprit, to determine where this missile was fired, whether it's from Ukrainian territory, Russian territory or Belorussian territory is all going to potentially matter," Center for Strategic and International Studies Europe program director Max Bergmann said.
The outcome of that investigation could have triggered NATO's principle of collective defence known as Article 5, kicking off deliberations on a potential military response.
"A deliberate and fatal attack on a NATO member could be considered a declaration of war," Flinders University international relations lecturer Jessica Genauer said.
"The accidental overshooting of a missile, whilst tragic and unacceptable, does not necessarily constitute an act of war."
Mr Bergmann said there were "many other steps that the NATO alliance can take to hit back or punish" whoever was responsible for the blast.
What is NATO Article 4?
It says the member countries can call meetings if they feel threatened.
Here's the full text:
"The Parties 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."
What is NATO Article 5?
It steps out how NATO member countries respond to an armed attack.
Here's the full text:
"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 and consequently 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.
"Any such armed attack and all measures taken as a result thereof shall immediately be reported to the Security Council. Such measures shall be terminated when the Security Council has taken the measures necessary to restore and maintain international peace and security."
Is this beginning of World War III?
Mr Bergmann doesn't think so.
"I doubt this will trigger Article 5 and that NATO will uniformly agree to sort of go ahead to launch an attack against Russia," she said.
"I don't think that will happen.
"I don't think this is going to precipitate World War III."
Is Australia part of NATO?
No.
But Australia has close ties with the alliance.
Australia is one of six countries granted NATO Enhanced Opportunities Partner status, and has a history of supporting NATO — participating in action in Afghanistan and Iraq.
Which countries are part of NATO?
There are 30 member countries. Here's the full list:
- Albania
- Belgium
- Bulgaria
- Canada
- Croatia
- Czechia
- Denmark
- Estonia
- France
- Germany
- Greece
- Hungary
- Iceland
- Italy
- Latvia
- Lithuania
- Luxembourg
- Montenegro
- Netherlands
- North Macedonia
- Norway
- Poland
- Portugal
- Romania
- Slovakia
- Slovenia
- Spain
- Türkiye
- United Kingdom
- United States
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