With Finland’s official membership in NATO taking effect on Tuesday, the military alliance’s border with Russia will expand by 832 miles — and Vladimir Putin is not happy about it. 

It was no longer a surprise that Finland would become NATO’s 31st member once Turkey ratified the country’s membership, and few at Helsinki are surprised by the Kremlin’s announcement about an apparently imminent  change to its Finland-facing force posture. 

Last month Finland’s defense minister, Antti Kaikkonen, called NATO membership “a defensive decision for Finland” and said the Finns “want to ensure our own security, peace, freedom.” He added: “I think that Russia will evaluate what kind of NATO member Finland will become, and, in accordance with this, it will possibly take countermeasures.”

As the Finnish blue cross flag is hoisted alongside the standards of all the other alliance members at the organization’s Brussels headquarters today, the general contours of some of those countermeasures were coming into sharper focus.

Speaking to the Kremlin mouthpiece RIA Novosti, the Russian deputy foreign minister, Aleksandr Gruško, said that “if troops and resources from other NATO member countries are deployed in Finland, in that case we will take additional measures to reliably ensure Russia’s military security.”

Mr. Gruško added that “we are strengthening our military potential in the direction of the west and northwest,” which indicates the Russian lands abutting the long border with Finland. 

Last year the Russian defense minister, Sergei Shoigu, said that he would oversee the formation of new divisions and units in Russia’s western military districts.

Yet most Finns seem unfazed by the Russian rhetoric. Finnish daily Iltalehti reported that the Russian announcement was expected. The newspaper quoted a Finnish professor of political science, Heikki Patomäki, who said that “at the moment, Russia doesn’t have any resources to build any big empire of its own here.” 

In addition, as the Sun’s Benny Avni reports, the Nordic country famous for its long, cold winters and plentiful, hot saunas also has no shortage of artillery power, should the need to use it ever arise.

In Mr. Patomäki’s estimation, it is because of Finland’s proximity to St. Petersburg as well as Russia’s northern Kola Peninsula that Moscow almost has to respond to NATO’s enlargement if only with some non-specific public-facing bluster. 

Of greater concern may be what is not being said. Something that Helsinki will have to be particularly attentive to in the weeks and probably years ahead is the threat of hybrid activity from across the long border. Last week Finland’s foreign minister, Pekka Haavisto, told reporters that “we must continue to monitor the situation in the airspace, on land and sea borders for any provocations. Cyber and hybrid impacts are also possible at the moment. It is good to be prepared for all this.”

And it has been preparing. The Finnish spy agency, SUPO, released a statement in recent days confirming successful measures against Moscow’s intelligence operations in Finland. SUPO’s director,  Antti Pelttari said that “the Russian intelligence station [in Finland] shrank to about half of its former size last year.” 

That was down largely to the expulsion of suspected Russian spies and severe restrictions on travel across the Russian-Finland border that Helsinki has imposed since Russia’s invasion of Ukraine. 

Mr. Peltarri said that “Russia is still seeking to station intelligence officers under diplomatic cover” and that Finland’s membership in NATO will increase Russia’s view of Finland as a target for intel and influencing operations. 

These are certainly interesting times for the Nordic nation of 5.5 million people, who are consistently ranked as among the happiest in the world. On Sunday, Finland’s main conservative party, the National Coalition, defeated the center-left Social Democratic party, leaving the 37-year-old prime minister, Sanna Marin, out in the cold. Ms. Marin courted controversy with a leaked video showing her dancing at a rowdy party but she was widely seen as constructively steering Finland toward NATO membership. 

Mr.  Haavisto traveled to Brussels one day ahead of the formal ceremony “with Finland’s NATO documents in his briefcase,” Finnish media reported. Finland’s president, Sauli Niinistö, will be there as well, of course, as will the alliance’s secretary-general, Jens Stoltenberg. According to Tuesday’s schedule, Turkey will hand its acceptance letter for Finland’s accession to Secretary Blinken at the NATO headquarters at Brussels.

The accession to NATO of Finland’s Scandinavian neighbor Sweden will not be in tandem as many had hoped, due to stubborn objections from Turkey.

Meanwhile at Helsinki the likely incoming center-right premier, 53-year-old Petteri Orpo, can be expected to stay the new course as all of northern Europe confronts a new security reality on the Continent that grows more challenging by the day.