“NATO membership would strengthen Finland’s security. As a member of NATO, Finland would strengthen the entire defence alliance,” Niinisto and Marin said in a joint statement.
“Finland must apply for NATO membership without delay," they said. "We hope that the national steps still needed to make this decision will be taken rapidly within the next few days.”
Russia reacted to the development with a warning. The Foreign Ministry said in a statement that if Finland joins NATO, it will “inflict serious damage to Russian-Finnish relations as well as stability and security in Northern Europe."
“Russia will be forced to take retaliatory steps of military-technical and other characteristics in order to counter the emerging threats to its national security," the ministry said.
“History will determine why Finland needed to turn its territory into a bulwark of military face-off with Russia while losing independence in making its own decisions,” it added.
The ministry’s statement follows Kremlin spokesman Dmitry Peskov’s comment earlier Thursday that Finland’s decision wouldn’t help stability and security in Europe. Peskov said that Russia’s response will depend on NATO’s moves to expand its infrastructure closer to the Russian borders.
Previously, the Kremlin had warned of “military and political repercussions” if Sweden and Finland decide to join NATO. Should they apply, there will be an interim period lasting from when an application has been handed in until all 30 NATO members’ parliaments have ratified it.
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In NATO member Estonia, which also borders Russia, Prime Minister Kaja Kallas tweeted that “history being made by our northern neighbors.” She pledged to support “a rapid accession process” for Finland into NATO.
Finland's announcement came a day after British Prime Minister Boris Johnson visited both Finland and Sweden to sign a military cooperation agreement.
The U.K. pledged on Wednesday to come to the aid of Sweden and Finland if the two Nordic nations came under attack.
During a joint news conference with Johnson and Niinisto in Helsinki, the Finnish head of state said Moscow could only blame itself should his nation of 5.5 million people become a NATO member.
“You (Russia) caused this. Look at the mirror,” Niinisto said pointedly Wednesday.
On Thursday, Niinisto tweeted that he spoke with Ukrainian President Volodymyr Zelenskyy about Finland's firm support for Ukraine and the country's intention to join NATO. Niinisto said that Zelenskyy “expressed his full support for it."
In 2017, Sweden and Finland joined the British-led Joint Expeditionary Force, which is designed to be more flexible and respond more quickly than the larger NATO alliance. It uses NATO standards and doctrine, so it can operate in conjunction with NATO, the United Nations or other multinational coalitions. Fully operational since 2018, the force has held a number of exercises both independently and in cooperation with NATO.
Russia’s invasion of Ukraine on Feb. 24 prompted Finland and Sweden to reconsider their historic tradition of military nonalignment and join the 30-member NATO. After Moscow launched its attack on Ukraine, public support in the two countries started to quickly shift toward membership in NATO, first in Finland and a bit later in Sweden.
The latest opinion poll conducted by Finnish public broadcaster YLE showed earlier this week that 76% of Finns are in favor of joining NATO, a big change from earlier years when only 20-30% of respondents favored such military alignment.
Speaking to European Union lawmakers Thursday as Niinisto's and Marin's announcement was made, Finnish Foreign Minister Pekka Haavisto said that “the war started by Russia jeopardizes the security and stability of the whole of Europe.”
Haavisto said that Russia’s unpredictable behavior is a serious concern for Finland, notably Moscow’s readiness to wage “high-risk operations” that could lead to many casualties, including among Russians themselves.
Should Finland become a NATO member, it would mean the biggest change in the Nordic country’s defense and security policy since World War II when it fought two lost wars against the Soviet Union. Along with Sweden, Finland joined the European Union in 1995 and has the longest border with Russia out of all the bloc's 27 members.
Swedish Foreign Minister Ann Linde tweeted that Finland’s announcement gave an “important message” and Danish Prime Minister Mette Frederiksen said that there were “strong messages" from Finland's president and prime minister.
During the Cold War, Finland stayed away from NATO to avoid provoking the Soviet Union, instead opting to remain a neutral buffer between the East and the West while maintaining good relations with Moscow and also with the United States.
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.
NATO officials say the Nordic duo's accession process could be done “in a couple of weeks.” The most time consuming part of the procedure – ratification of the country’s protocol by the 30 NATO member countries – could even be completed in less time than the four months or so that it took West Germany, Turkey and Greece to join in the 1950s, when there were only 12 members to ratify their applications.
“These are not normal times,” one NATO official said this week, discussing the possible applications of Finland and Sweden. The official was briefing reporters about the accession process on condition that he not be named as no application has been made by the two countries.
Lorne Cook in Brussels, and Jan M. Olsen in Copenhagen, Denmark, contributed to this report.