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-kilometer (800-mile) border with Russia — and its neighbor Sweden to drop decades of military non-alignment.

How is the accession being marked?

On Tuesday afternoon, 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.

During the ceremony, Blinken took a jab at Russian President Vladimir Putin.

"I'm tempted to say this is maybe the one thing that we can thank Mr. Putin for because he once again here precipitated something he claims to want to prevent by Russia's aggression, causing many countries to believe that they have to do more to look out for their own defense and to make sure that they can deter possible Russian aggression going forward," Blinken said just before accepting the documents that made Finland's membership official.

NATO Secretary-General Jens Stoltenberg echoed the sentiment.

"President Putin wanted to slam NATO's door shut. Today we show the world that he failed, that aggression and intimidation do not work," Stoltenberg said.

Finnish security expert Henri Vanhanen told DW that the invasion of Ukraine last year marked a start of a new era.

"The masks have been removed and we sort of see the real face of Putin right now," he said. "And I think in the foreseeable future, it will be difficult to see a sort of return to any sort of normal relationship with the Russians, at least as long as the current administration is in place."

UK wants Sweden to join next

Helsinki will work "relentlessly" to secure Sweden's membership of NATO, pledged Finnish President Sauli Niinisto on Tuesday.

"Finland's membership is not complete without Swedish membership," Niinisto said in a statement.

The United Kingdom also urged the military alliance to admit Sweden next.

"All NATO members now need to take the steps necessary to admit Sweden too, so we can stand together as one Alliance to defend freedom in Europe and across the world," Sunak said in a statement after Finland officially joined.

Finland's accession came as NATO foreign ministers met 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 in 1949.

Russia's NATO frontier doubles in length

Russia's Foreign Ministry said that, by joining the alliance, Finland had lost "its self-identity and any independence."

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.

"This forces us to take countermeasures ... in tactical and strategic terms," said Kremlin spokesman Dmitry Peskov.

Turkey and Hungary, for different reasons of their own, initially 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 Stoltenberg pointed out that  "Sweden will also be safer as a result" of Finland's membership. He added that he expected Sweden would be able to join soon.

rc/dj (AFP, Reuters)