In the near term, the Finns are more worried about a very different form of warfare — weaponized migration. About 60 miles north of Vaalimaa, Finland has begun to install its first border fence.

In late 2015 and early 2016, Finland experienced a surge of asylum-seeking migrants crossing the Russian border, most of them from third countries. Finnish officials saw the hand of Moscow, which has repeatedly directed migrants into European countries in an apparent effort to destabilize their politics.

“The impression that someone is organizing and regulating things on the Russian side is probably true,” Finland’s foreign minister, Timo Soini, told the country’s state broadcaster at the time. “It is quite obvious that activity like this is a managed effort.”

The Finns were caught off guard. “Never in my wildest dreams did I anticipate that we would have, for example, Bangladeshis coming with bicycles to a high north border crossing when the sun doesn’t come up at all and it’s minus 20-25 degrees Celsius,” Mr. Pitkäniitty said, or minus 4 to minus 13 degrees Fahrenheit.

Despite that experience, Mr. Pitkäniitty said that he and his colleagues maintain cordial and professional relations with their Russian counterparts across the border. The two sides communicate regularly, he said.

“When we talk to the Russians we try to avoid politics,” Mr. Pitkäniitty said. “There is no point in arguing. You just end up in a dispute that does not allow for solutions.”

For years, he said, acceptable conversation topics with the Russians included fishing, hunting and sports. “Now we have to exclude sports, because they do not participate in international sports anymore,” Mr. Pitkäniitty said. “So it’s fishing and hunting you can safely talk about with the Russian officers.”

At the same time, “I know that they will not hesitate to shoot me in the back if ordered to do so,” he added. “Just as I would do the same to them.”

John Ismay contributed reporting from Washington, D.C.