Brussels [Belgium]: Finland became the 31st member of the NATO military alliance on Tuesday amid warnings of "countermeasures" from the Kremlin.
Russia's invasion of Ukraine last year prompted Finland — which has a 1,300-kilometre (800-mile) border with Russia — and its neighbor Sweden to drop decades of military non-alignment.
How is the accession being marked?
Finland's foreign minister handed the accession papers to US Secretary of State Antony Blinken, the keeper of NATO's founding treaty.
Finland's blue and white flag was then set to be hoisted alongside those of its new allies, alphabetically placed between the flags of Estonia and France, in front of NATO headquarters in Brussels.
Finland's accession came as NATO foreign ministers met Tuesday at the alliance's headquarters in Brussels for a two-day summit to discuss the ongoing Russian invasion of Ukraine. The meeting coincides with the 74th anniversary of the alliance's founding.
Russia's foreign ministry said that, by joining the alliance, Finland had lost "its self-identity and any independence."
"This forces us to take countermeasures ... in tactical and strategic terms," said Kremlin spokesman Dmitry Peskov.
Russia's NATO frontier doubles in length
The enlargement of NATO's border with Russia has "profoundly changed the situation" in northern Europe, the ministry said. Finland's membership doubles Russia's border with NATO, the world's largest military alliance.
Türkiye and Hungary, for different reasons of their own, had held up Finland's effort to join. Sweden's progress remains blocked.
Achieving ratification in well under a year still makes Finland's membership process the fastest in the alliance's recent history.
Membership for Sweden is still dependent on Turkish ratification due to several sticking points, but NATO Secretary General Jens Stoltenberg pointed out "Sweden will also be safer as a result" of Finland's membership. He added that he expected Sweden would be able to join soon.