Finland, which shares a lengthy land border with Russia, has announced plans to join Nato - despite threats from its hostile neighbour that membership of the North Atlantic Treaty Organisation could make it a target.
The move came just hours before Sweden's governing party backed a plan to join the trans-Atlantic alliance amid Russia's war in Ukraine.
Finland's president Sauli Niinisto said a "new era begins" with his country's decision as he hailed the "historic day".
He made the announcement alongside Prime Minister Sanna Marin at a joint news conference at the Presidential Palace in Helsinki.
The Finnish Parliament is expected to endorse the decision in coming days, but it is considered a formality.
A formal membership application will then be submitted to Nato headquarters in Brussels, most likely at the some point next week.
Dmitry Polansky, first deputy representative of Russia to the UN, said in an interview with the British conservative magazine UnHerd this week, that Swedish and Finnish membership would see those nations "become a target(s) - or possible target(s) - for a strike".
“Nato is a very unfriendly bloc to us — it is an enemy and Nato itself admitted that Russia is the enemy. It means that Finland and Sweden all of a sudden, instead of neutral countries, become part of the enemy and they bear all the risks.”
Sweden has already taken steps toward joining the alliance, while Georgia's bid is again being discussed despite dire warnings from Moscow about the consequences if its neighbour becomes part of Nato.
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Deputy-Secretary General Mircea Geoana said “Finland and Sweden are already the closest partners of Nato,” and expects allies to view their applications positively.
German Foreign Minister Annalena Baerbock said her country and others made clear during a dinner late Saturday that they would be willing to fast-track the national ratification process for Finland and Sweden.
“If these two countries are deciding to join, they can join very quickly,” she said.
Denmark's foreign minister dismissed suggestions that objections from Russian President Vladimir Putin could hinder the alliance from letting in new members.
“Each and every European country has a fundamental right to choose their own security arrangement," Jeppe Kofod told reporters.
Why does it matter that Finland intends to join Nato?
The Kremlin has warned of “military and political repercussions” if Finland and Sweden decide to join the alliance, having previously argued that Nato support for Ukraine constituted a growing threat on its borders.
Currently, five countries that border Russia are Nato members, but only six percent of its land borders touch Nato countries.
Should Finland and Sweden apply for Nato membership, there will be an interim period lasting from when an application has been handed in until all 30 Nato members’ parliaments have ratified it.
What happens if Finland becomes a member of Nato?
Should Finland become a Nato member, it would mean the biggest change in the Nordic country’s defence and security policy since World War II when it fought two lost wars against the Soviet Union.
Finland stayed away from Nato during the Cold War to avoid provoking its neighbour, instead opting to remain a neutral buffer between East and West.
Nato Secretary-General Jens Stoltenberg has said the military alliance would welcome Finland and Sweden - both of which have strong, modern militaries - with open arms, and expects the accession process to be speedy and smooth.