Finland and Sweden have announced they intend to join the NATO military alliance, a major shift away from their policy of military non-alignment.
Security concerns over Russia's invasion of Ukraine have changed the calculus for Finland and Sweden, and caused other traditionally "neutral" countries to rethink what that term really means for them.
However, it has also exposed division between NATO members, with Turkey opposing a move for the two Scandinavian countries to join the alliance.
Russian President Vladimir Putin said the expansion of NATO into Finland and Sweden would "of course, give rise to our reaction in response".
So how significant would acceptance of Finland and Sweden into NATO be?
Why does it matter if Finland and Sweden join NATO?
The Northern Atlantic Treaty Organisation, or NATO, was formed in the aftermath of World War II to provide collective security against the Soviet Union and is currently the world's largest military alliance.
Its backbone is Article 5, which states that if one ally is a victim of an attack, it is considered an attack against all members, giving smaller countries protection and access to equipment.
NATO, since its inception in 1949, has grown to include other European states, most notably in 2004 when a swath of former Soviet eastern European countries joined the alliance.
Russia maintains that, after the Cold War, NATO had promised it would not expand eastwards to the former Soviet states.
Russian President Vladimir Putin has justified his military's invasion of Ukraine over fears that NATO would further expand to accept Kyiv into its alliance.
However, it appears Mr Putin's hopes of fending off further NATO expansion in Europe have been dashed.
Neighbouring Finland and Sweden have traditionally remained non-aligned in order to maintain good relations with Russia.
However, public opinion in those countries shifted dramatically towards supporting NATO membership after Russia invaded Ukraine.
If Finland and Sweden are accepted into the alliance, it will increase the number of member states to 32 and be the latest NATO expansion since 2009, when Albania and Croatia joined.
Speaking to a Russian-led military alliance of six former Soviet states, Mr Putin said Moscow did "not have a problem" with Sweden or Finland applying for NATO membership, but that "the expansion of military infrastructure onto this territory will, of course, give rise to our reaction in response".
Why is Turkey against them joining?
Turkey's President, Recep Tayyip Erdogan, said he could not allow Finland and Sweden to become NATO members because of their perceived inaction against exiled Kurdish militants.
Mr Erdogan accused the two countries of refusing to extradite "terrorists" wanted by his country.
"Neither country has an open, clear stance against terrorist organisations," Mr Erdogan said, in an apparent reference to Kurdish militant groups such as the banned Kurdistan Workers' Party (PKK).
Sweden has welcomed hundreds of thousands of refugees from the Middle East in recent decades, including ethnic Kurds from Syria, Iraq and Turkey.
Turkey's objection has taken many officials by surprise and, if Mr Erdogan follows through on his threat, this could prevent Finland and Sweden joining NATO.
That's because all 30 current NATO countries must unanimously agree to open the door to new members.
Swedish officials said they would dispatch a team of diplomats to Ankara to discuss the matter, but Mr Erdogan suggested they were wasting their time.
"Are they coming to try and convince us? Sorry, don't wear yourselves out," Mr Erdogan said.
Who is left as a 'neutral' country in Europe and who might be next?
While Russia's invasion of Ukraine has switched some staunch supporters of neutrality in Finland and Sweden towards accepting NATO membership, it appears unlikely other neutral European countries will follow.
Arguably the most renowned neutral country in Europe, Switzerland, has enshrined neutrality into its constitution and Swiss voters decided decades ago to stay out of the EU.
There's little chance that Switzerland will stray further from its neutrality: Its government has already asked Germany not to pass along Swiss military equipment to Ukraine.
Austria's neutrality is a key component of its modern democracy: As a condition of Allied forces leaving the country and its ability to regain independence in 1955, Austria declared itself militarily neutral.
Chancellor Karl Nehammer said his country had no plans to change its security status.
Ireland's Prime Minister, Micheál Martin, summed up the country's position earlier this year as: "We're not politically neutral, but we're militarily neutral."
The small Mediterranean island of Malta is officially neutral, hewing to a policy of "non-alignment and refusing to participate in any military alliance".
And the President of Cyprus, Nicos Anastasiades, said on Saturday that it was "much too early" to even contemplate a move to join NATO, that would invariably meet rival Turkey's strong opposition.
What are the next steps for the two nations to join?
NATO officials say the membership procedure could be completed "in a couple of weeks".
However, the most time-consuming part — ratification of their accession protocols by the alliance's 30 member countries, sometimes involving parliaments — could take months.
Firstly, a request to join must be submitted. It usually comes in the form of a letter from a government minister or leader.
NATO then assesses that request. That's done in a sitting of the North Atlantic Council (NAC) of the 30 member countries, probably at ambassadorial level.
The NAC decides whether to move toward membership and what steps must be taken to achieve it.
If the NAC gives a green light, accession talks are held. These are likely to be completed in just one day. The steps are fairly straight forward.
The candidate is asked to commit to uphold Article 5. It would have to commit to spending obligations concerning the NATO in-house budget.
After more discussions with all member states, the NAC makes a final decision about whether an application is successful.
If Sweden and Finland are successful, a small ceremony would be held, giving a symbolic and legal form to this part of the membership process.
The protocol is then sent to capitals for ratification, according to the 30 national procedures, some of which require parliamentary approval.
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