Early in the day on December 5, 2017, Frode Berg — a 62-year-old pensioner and former border guard from Norway — posted a photo of a snow-covered Red Square on his Facebook page with the caption “Christmas Time in Moscow!”

Berg had left his home in Kirkenes, a Norwegian town of about 3,500 people near the country’s 121-mile Arctic border with Russia, a day earlier for a weekend trip to the Russian capital to visit friends and do some Christmas shopping. But he never returned.

Berg was arrested by agents from Russia’s F.S.B., the successor agency to the K.G.B., who said they found an envelope on him holding 3,000 euros in cash. They accused him of involvement in an elaborate spying operation, dating back to 2015, to obtain information about Russia’s nuclear submarine fleet in the far north.

Boots on the ground: with Britain sending 800 commandos to join U.S. Marines already there, NATO’s footprint in Norway is expanding (Photo: US Marine Corps)

Ten months later, Berg remains incarcerated in Moscow’s high-security Lefortovo prison, still not officially charged, but facing the possibility of 20 years in jail. Relations between Russia and Norway — a NATO member — have plunged to their lowest point since the Cold War. But many suspect there’s another level to this Arctic spy drama, and that Russia may have been just as interested in sowing distrust and divisions in its Nordic neighbor —prompted partly by the deployment of U.S. troops there — as closing down any spying on its undersea activities.