Finland has officially become a member of NATO after concerns over Russia’s aggression.
The Kremlin has said it could be forced to take “counter measures” to ensure its own border’s security in response.
Finland applied to join NATO in May 2022, setting aside years of military nonalignment to seek protection under the organisation’s security umbrella after Russia’s invasion of Ukraine.
It shares a 1,340km (832-mile) border with Russia, so its entry will more than double the size of NATO’s border with the nation.
Finland’s President Sauli Niinistö said in a statement: “Finland has today become a member of the defence alliance NATO. The era of military non-alignment in our history has come to an end. A new era begins.
“Each country maximises its own security. So does Finland. At the same time, NATO membership strengthens our international position and room for manoeuvre. As a partner, we have long actively participated in NATO activities. In the future, Finland will make a contribution to NATO’s collective deterrence and defence.
“Membership of the Alliance provides security for Finland. Finland, on the other hand, provides security for the Alliance. Finland, committed to the security of all NATO member states, will be a reliable ally that strengthens regional stability.
“Finland’s membership is not targeted against anyone. Nor does it change the foundations or objectives of Finland’s foreign and security policy. Finland is a stable and predictable Nordic country that seeks peaceful resolution of disputes.”
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The move is a strategic and political blow to Russian President Vladimir Putin, who has long complained about NATO’s expansion toward Russia.
Kremlin spokesman Dmitry Peskov called Helsinki’s move to join the bloc an “encroachment” on Russia’s security and said the structure of NATO was hostile towards Russia.
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A ceremony was held outside the organisation’s Brussels headquarters to raise Finland’s blue and white flag among those of its partners.
Earlier, NATO secretary-general Jens Stoltenberg told reporters in Brussels that “there will be no NATO troops in Finland without the consent of Finland”.
But he refused to rule out the possibility of holding more military exercises there and said that NATO would not allow Russia’s demands to dictate the organisation’s decisions.
The ceremony has fallen on NATO’s birthday, the 74th anniversary of the signing of its founding Washington Treaty on 4 April 1949. It also coincides with a meeting of the alliance’s foreign ministers.
It is the first enlargement of NATO since North Macedonia joined the alliance in 2020.
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Turkey was the last of NATO’s 30 members to accept Finland’s application – which ends the country’s decades of military non-alignment. The document officially enshrining Turkey’s decision was sent to US Secretary of State Antony Blinken before the ceremony.
Finland then gave Mr Blinken its own final texts, making its membership official. The US State Department is the repository of NATO texts concerning membership.
Turkey is still blocking the approval of Sweden joining NATO, with the government saying Stockholm has so far failed to sufficiently crackdown on similar groups.