Finnish President Sauli Niinisto, right, and Russian President Vladimir Putin leave after a news conference at the Black Sea resort of Sochi on Aug. 15, 2014. (IVAN SEKRETAREV/AFP/Getty Images)

Wedged hard against Russia’s northwestern border, peaceable Finland has long gone out of its way to avoid prodding the nuclear-armed bear next door.

But now the bear is provoking Finland, repeatedly guiding military planes into Finnish airspace and deploying submarines and helicopters to chase after Finnish research vessels in international waters.

The incidents are part of a pattern of aggressive Russian behavior that has radiated across Europe but that has been especially unnerving for countries such as Finland that live outside the protective bubble of NATO.

(Related: Finland’s leaders see peril in standoff between Russia and the West)

As Russian-backed separatists have eviscerated another non-NATO neighbor this year — Ukraine — Finnish leaders have watched with growing alarm. They are increasingly questioning whether the nonaligned path they navigated through the Cold War can keep them safe as Europe heads toward another period of dangerous standoffs between West and East.

Map of incidents involving Russian and NATO forces

“We have a long history with Russia — not that peaceful all the time. So everything the Russians are doing, surely the Finns notice and think very carefully about what that might mean,” Finnish President Sauli Niinisto said in an interview at his coastal residence in this capital city, just a two-hour drive from the Russian border.

In the case of the recent air incursions, he said, the message was clear: “They were testing how we’d react.”

Niinisto said Finland’s ­response — scrambling American-made F-18 Hornet fighter jets to intercept the Russian planes — was strong enough to ward off further Russian aggression.

But the palpable anxiety in this country that many in the West consider a model of progressive and stable democratic governance reflects how unsettled Europe has become since Russia’s annexation of Crimea in March.

Many in Helsinki are convinced that Russia will not remain deterred for long and say Finland needs to fundamentally rethink elements of its security policy that have been bedrock principles for decades.

“People used to think European Union membership was enough to protect us,” said Tarja Cronberg, a former member of the Finnish and European parliaments. “But now that’s being questioned.”

Finnish Prime Minister Alexander Stubb has argued that the country needs to join NATO, and a growing share of the public seems to agree, although the ­issue remains hugely sensitive here.

Even Finns who favor membership in the alliance acknowledge that joining would be a gamble, with Russia threatening to disrupt the peace and prosperity this country has long enjoyed if Finland makes a sudden lurch toward the West.

“It’s going in a terrifying direction,” said Elisabeth Rehn, a former Finnish defense minister who favors NATO membership. “It’s only been 100 years since we gained our independence from Russia. Crimea was a part of Russia, too. Will they try to take back what belonged to them 100 years ago?”

Rehn said she doubts Russia would go that far but said the fear of Russian military aggression is real.

“We don’t have a normal relationship with Russia,” said Rehn, who as a child watched boys from her village come home in coffins after battling Soviet troops in World War II. “We all like the Russians. They sing the same melancholic songs that we do. But we are afraid of their leadership.”

Finland is hardly the only one. Next door in Sweden, the country’s armed forces mounted their largest operation since the Cold War last month to hunt for a suspected Russian submarine. Swedish defense chiefs may have viewed the hunt as a chance to compensate for a conspicuous lapse last year, when Russian warplanes simulated an air assault on Stockholm and the Swedish military failed to react.

Like Finland, Sweden has remained outside NATO even as other Baltic nations — including Estonia, Latvia and Lithuania — have joined.

But NATO membership has not protected those countries from Russian probing, which has extended in recent months across Europe, as far afield as Portugal.

A report issued this month by the European Leadership Network, a London-based think tank, documented nearly 40 incidents that together “add up to a highly disturbing picture of violations of national airspace, emergency scrambles, narrowly avoided midair collisions, close encounters at sea, simulated attack runs, and other dangerous actions happening on a regular basis over a very wide geographical area.”

The report concluded that Russia was not trying to provoke a conflict but that the behavior “could prove catastrophic” because of the risk of unintended escalation.

Russian officials have brushed off the significance of the breaches. Speaking in Washington this month, Sergey Kislyak, the Russian ambassador to the United States, said they were the result of an increase in military training flights.

Such explanations are viewed with extreme skepticism here in Finland, where the country’s airspace was violated in August three times in one week. The breaches came just days before Finland forged greater cooperation with NATO at the alliance’s summit in Wales, and leaders here say they were a clear signal from Moscow that Finland dare not go any further.

Finland, a nation of 5 million people, has an 800-mile border with Russia, which has a population nearly 30 times as large. Moscow would undoubtedly consider NATO membership a direct challenge in territory that was marked as neutral space during the Cold War. A Kremlin adviser said earlier this year that if Finland and Sweden join NATO, it could lead to World War III.

Finland already has taken substantial steps toward the West, joining with fellow European Union members in imposing sanctions on Russia for its aggression in Ukraine. But Finland also has upped its trade ties with Russia and leans heavily on its eastern neighbor for oil and gas.

Finnish Foreign Minister Erkki Tuomioja said in an interview that Russia benefits from its relations with Finland and has no incentive to start a war.

“From a Russian point of view, it’s the most stable and least problematic frontier they have, and I believe they want to keep it that way as long as they have no reason to believe that the Finnish territory would be used for hostile action,” Tuomioja said.

Still, Finland is taking few chances, boosting defense spending and border patrols while tightening relations with Sweden. “Our thinking has always been that if Russia comes here, it might mean the end of us, but it would cost Russia so much they would think twice,” said Hanna Smith, a Russia scholar at the University of Helsinki.

Niinisto suggested that logic still stands, pointing to the ­country’s quarter-million-strong armed forces, which are supported through conscription, and noting that “250,000 men is something you have to at least take notice of.”

But he also doesn’t rule out calling in reinforcements — and considers the NATO option necessary to retain to keep Russia at bay.

“I’m not against NATO. I see that as a possibility,” said Niinisto, who regularly talks to senior Russian leaders, including President Vladimir Putin. “It has to be an open choice, and that is important, because it is also part of our balance.”

Karla Adam in London and
Karen DeYoung in Washington contributed to this report.