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.
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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.
It falls 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.
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.