WASHINGTON >> The first surprise, for the Finnish conscripts and officers taking part in a NATO-hosted military exercise in the Arctic this spring: the sudden roar of a U.S. Marine helicopter assault force, touching down in a field right next to the Finns’ well-hidden command post.

The second surprise: Spilling out of their field headquarters, the Finnish Signal Corps communications workers and others inside routed the U.S. Marines — the Finns’ designated adversary in the NATO exercise and members of America’s professional and premier expeditionary force — in the mock firefight that followed.

Finnish camouflage for the Arctic snow, scrub and scree likely had kept the Americans from even realizing the command post was there when they landed, Finnish commander Lt. Col. Mikko Kuoka suspected. “For those who years from now will doubt it,” Kuoka, modestly stunned by the outcome of the random skirmish, wrote in an infantry-focused blog recording the outcome, of an episode he later confirmed for The Associated Press. “That actually happened.”

As the exercise made clear, NATO’s addition of Finland and Sweden — what President Joe Biden calls “our allies of the high north” — would bring military and territorial advantages to the Western defense alliance. That’s especially so as the rapid melting of the Arctic from climate change awakens strategic rivalries at the top of the world.

In contrast to the NATO expansion of former Soviet states that needed big boosts in the decades after the Cold War, the alliance would be bringing in two sophisticated militaries and, in Finland’s case, a country with a remarkable tradition of national defense. Both Finland and Sweden are in a region on one of Europe’s front lines and meeting places with Russia.

Finland, defending against Soviet Russia’s invasion on the eve of World War II, relied on fighters on snowshoes and skis, expert snow and forest camouflage, and reindeers transporting weapons.

Russian President Vladimir Putin’s invasion of Ukraine in late February, along with his pointed reminder about the Kremlin’s nuclear arsenal and his repeated invocation of broad territorial claims stemming from the days of the Russian Empire, have galvanized current NATO nations into strengthening their collective defenses and bringing on board new members.

Finland — until 1917 a grand duchy in that empire — and Sweden abandoned longtime national policies of military nonalignment. They applied to come under NATO’s nuclear and conventional umbrella and join what is now 30 other member states in a powerful mutual defense pact, stipulating that an attack on one member is an attack on all.