Finland and Sweden are poised to end decades of neutrality by joining NATO, a dramatic evolution in European security and geopolitics sparked by Russia's invasion of Ukraine.

The two Nordic nations had long kept the military alliance at an arm's length, even while eying Russia to their east with caution.

But Moscow's assault on Ukraine has sparked renewed security concern across the region, and the leaders of each country have signaled their desire to join the bloc after more than 75 years of military nonalignment.

NATO has what it calls an “open door policy” on new members — any European country can request to join, so long as they meet certain criteria and all existing members agree.

A country does not technically “apply” to join; Article 10 of its founding treaty states once a nation has expressed interest, the existing member states “may, by unanimous agreement, invite any other European State in a position to further the principles of this Treaty … to accede.”

NATO diplomats told Reuters ratification of new members could take a year, as the legislatures of all 30 current members must approve new applicants.

Both Finland and Sweden already meet many of the requirements for membership, which include having a functioning democratic political system based on a market economy; treating minority populations fairly; committing to resolve conflicts peacefully; the ability and willingness to make a military contribution to NATO operations; and committing to democratic civil-military relations and institutions.

The process was not without hurdles; Turkey's President Recep Tayyip Erdogan said Friday he was not looking at both countries joining NATO “positively,” accusing them of housing Kurdish “terrorist organizations.” But on Tuesday, he threw his support behind the nations’ bids at the NATO summit in Madrid, Spain.

The United States and the United Kingdom have both expressed their support for their membership bid.

What does NATO membership entail?

The reason most countries join NATO is because of Article 5 of the North Atlantic Treaty, which stipulates all signatories consider an attack on one an attack against all.

Article 5 has been a cornerstone of the alliance since NATO was founded in 1949 as a counterweight to the Soviet Union.

The point of the treaty, and Article 5 specifically, was to deter the Soviets from attacking liberal democracies lacking military strength. Article 5 guarantees the resources of the whole alliance — including the massive US military — can be used to protect any single member nation, such as smaller countries who would be defenseless without their allies. Iceland, for example, has no standing army.

Former Swedish leader Carl Bildt told CNN he doesn't see new big military bases being built in either country should they join NATO. He said joining the alliance would likely mean more joint military training and planning between Finland, Sweden and NATO's 30 current members. Swedish and Finnish forces could also participate in other NATO operations around the globe, such as those in the Baltic States, where several bases have multinational troops.

It's worth noting Russia has lambasted the decision by Finland and Sweden to join NATO. Its deputy Foreign Minister Sergei Ryabkov said Monday the move would be a “mistake” with “far-reaching consequences,” according to state news agency TASS.

Russia currently shares about 755 miles of land border with five NATO members, according to the alliance. Finland's accession would mean a nation with which Russia shares an 830-mile border would become formally militarily aligned with the United States.

The addition of Finland and Sweden would also benefit the alliance, which would frustrate Russia. Both are serious military powers, despite their small populations.

Russian President Vladimir Putin said Monday “Russia has no problems with these states,” adding the expansion of NATO “does not pose a direct threat to Russia.”

“But the expansion of military infrastructure into this territory will certainly cause our response,” he added at the Collective Security Treaty Organization in Moscow. “We will see what it will be based on the threats that will be created for us.”