• Finland and Sweden are set to apply for membership in NATO, the countries said Sunday.
• It's a historic move for the Nordic nations, which are known for their policies of military neutrality.
• Finland shares an 830-mile border with Russia; if it joins the military alliance, the land border that Russia shares with NATO territories would roughly double. Sweden does not have a land border with Russia, however it does share a maritime border with the country.

Finland and Sweden are both set to apply for membership in NATO, the countries said Sunday, in a historic move for the Nordic countries which are known for their policies of military neutrality.

In a press conference alongside Finland's Prime Minister Sanna Marin, the country's President Sauli Niinisto said: "Today, we, the president and the government's foreign policy committee, have together decided that Finland ... will apply for NATO membership."

He added that being a member of the military alliance will "maximize" Finland's security after Russia's unprecedented invasion of Ukraine in February.

Marin described that move to apply as an "important decision" based on a "strong mandate."

"We hope that the parliament will confirm the decision to apply for NATO membership during the coming days," she added.

Marin said Finland has been in close contact with NATO and its members over the decision. Last week, Marin and Niinisto said the country should apply to join NATO "without delay."

Shortly after Finland's announcement, Swedish Prime Minister Magdalena Andersson said she was also backing an application to join NATO. It comes after her Swedish Social Democratic Party relinquished its historic opposition to membership of the alliance, given the ongoing aggression of Russia in Ukraine.

"Today the Swedish Social Democratic Party took a historic decision to say yes to apply for a membership in the NATO defense alliance. The Russian invasion of Ukraine has deteriorated the security situation for Sweden and Europe as a whole," Ann Linde, Sweden's foreign affairs minister, said on Twitter.

Speaking to CNBC Sunday, Linde noted how Russia had not only invaded its neighbor Ukraine — but that it had conducted war crimes, targeting civilian infrastructure with the "bombing of schools and hospitals and theaters."

"This has made us take the decision that we will not be secure without applying for membership of NATO," she added.

Russia has repeatedly denied targeting civilian infrastructure, despite vast evidence to the contrary.

The formal application to join NATO is expected from both countries in the coming days.

'Radical change' in policy

Finland shares an 830-mile border with Russia; if it joins the military alliance, the land border that Russia shares with NATO territories would roughly double. Sweden does not have a land border with Russia, however it does share a maritime border with the country.

Given this proximity, there is a risk the move from Helsinki and Stockholm could spark aggression from Russia, where President Vladimir Putin has repeatedly expressed his opposition to NATO's enlargement.

Last week, Russia's foreign ministry said Finland joining NATO would be a "radical change" in the country's foreign policy. "Russia will be forced to take retaliatory steps, both of a military-technical and other nature, in order to stop threats to its national security arising," it said in a statement.

Finland's Niinisto said he spoke to Putin on Saturday and informed him of his country's decision. Speaking to CNBC Sunday, Sweden's Linde said she was hopeful there would not be an escalation of tensions between Russia and the Nordic region.

"During the period of transition, before Sweden and Finland get the full membership, there will be a heightening of tension in our area. We also foresee more military troops close to our borders," she added.

Russia has land borders with 14 countries and five of them are NATO members: Latvia, Estonia, Lithuania, Poland and Norway.

Finland and Sweden have both been reviewing their security policies following Russia's invasion of Ukraine, which showed the Kremlin is willing to attack a neighboring nation. Finland, for example, has been invaded in the past — in 1939, the Soviet Union attacked Finland in what became known as the Winter War.

Opposition from Turkey

One potential stumbling block to both countries joining the alliance is Turkey, the NATO member with the second-largest military after the U.S.

Ascension for a new member state requires consensus approval from all existing members.

The country's President Recep Tayyip Erdogan said Thursday: "We don't hold positive views" on Finland and Sweden's potential membership. Sweden is expected to follow Finland and also apply to join NATO in the near future.

Earlier Sunday, NATO's deputy secretary general struck a confident tone on Finland and Sweden potentially joining the group.

Speaking to reporters in Berlin, Mircea Geoana said the two countries were already the closest partners of NATO.

"I am confident that if these two countries will decide, in the next few days I understand, to seek membership in NATO, that [we] will be able to welcome them and to find all conditions for consensus to be met," he said.

On Turkey, Geoana added: "They expressed concerns that are addressed and discussed in between friends and allies."

NATO — or the North Atlantic Treaty Organization — was founded in 1949 by the U.S., Canada, and several Western European nations to provide collective security against modern Russia's forebearer, the Soviet Union.

Ever since its foundation, the alliance has had a thorny relationship with the Soviet Union throughout the Cold War and, following its collapse in 1991, the Russian Federation.

— CNBC's Natasha Turak contributed to this report