The Finnish Defense Forces sent out an urgent message: We are being invaded. We need help.

Hundreds of American troops — part of a new Arctic division — boarded planes in Fairbanks, Alaska. Their flight curved over the North Pole and landed at Rovaniemi Airport, in northern Finland. The soldiers quickly unpacked their M-4 assault rifles, rocket tubes and belt-fed machine guns and deployed to the quiet snowbound forests, dressed in Arctic whites and vaporproof boots.

This was all just a drill, launched in mid-February. But the scenario is believed to be increasingly possible. As climate change melts ice across the Arctic, this part of the world, once so remote and forgotten, is becoming more accessible and more contested. The world’s major militaries — American, Russian, Chinese and European — are all training for a winter war.

“It’s really only been the past five or six years that everybody’s moved on from the global war on terror,” said Robert McBride, a Canadian brigadier general helping oversee the war game.

“Arctic nations are starting to understand the strategic importance,” he said. “The Arctic now has come to pre-eminence.”

The recent drill played out at a strange time in global affairs. President Trump has been leaning away from NATO and getting friendly with Russia, and European leaders are seriously discussing how to create their own defense industry should America abandon them, something unthinkable just a few months ago.

But on this frozen ground, at least, American military cooperation and the perception of Russia as a widening threat appeared unchanged.

In Finland, which fought the Soviet Union during World War II, Russia remains the once and future enemy. “There’s an old Finnish saying,” explained Janne Kuusela, a Finnish defense official, “Russia will take what’s not nailed to the wall.”

America’s relationship with Finland, one of NATO’s newest members, seems solid. President Trump recently played golf with its president, Alexander Stubb. Afterward he praised Mr. Stubb’s golf skills and said, “I look forward to strengthening the partnership.”

The two sides worked well together during the battle in the snow. The Finns were nimble attackers, zigzagging through the woods on long, narrow skis. The Americans set up machine guns on small round hilltops and dug foxholes in the snow. Both sides said Arctic warfare was different.

“It’s kind of like operating in space,” said Christopher Brawley, an American colonel.

“No one’s coming to help you,” he said. “And the environment will kill you.”

The cardinal rule is staying dry. Jackson Crites Videman, a Finnish soldier, recounted a grueling test that Finnish soldiers must pass. With all their gear on, including skis, they plunge through an ice hole cut into a river and have to scamper out — without freezing or drowning.

The day he did it was minus 36 degrees Celsius (roughly minus 33 Fahrenheit). He had about two minutes to strip off drenched clothes and change into new ones before frostbite set in. And his hands stopped working.

“Your friends have to help you with the zippers,” he explained.

Mr. Crites Videman, who is half-Finnish, half-American, was drafted into the Finnish military a few months ago. Finland is one of the few Western democracies with mandatory conscription. It’s a country with a small population, 5.6 million, and NATO’s longest border with Russia — 830 miles — and it recently raised defense spending.

Finnish defense experts say that tens of thousands of Russian troops used to be based near that border but were decimated in the Ukraine war. The Finns believe it will take five to 10 years before they become a threat again. Before the Ukraine war, the Finns say, Russia was investing in its Arctic forces, modifying tanks to operate better in the cold and designing new troop transporters.

The United States has been doing the same. In 2022, it designated the 11th Airborne, based in Alaska, as its first and only Arctic division. The division is experimenting with new uniforms and different combat skis.

During the exercise, the Americans showed off a new tracked vehicle that chewed its way up icy slopes, some quite steep. The soldiers lived off special cold weather rations, higher in calories, because of all the energy drained from trying to stay warm.

Warfare during the Arctic summer isn’t much easier. First, there is the issue of the sun. It never sets. So the advantage that night-vision equipment gives Western militaries is eliminated.

When the ice melts, the land turns incredibly mushy. Upper Finland, for example, is covered in thick forests, small mountains, marshes, rivers, lakes and bogs, making it very difficult to navigate. In many ways, it’s easier to move in winter.

During the war game, the attackers exploited the frozen rivers like highways. Soldiers dashed across them, guns strapped to their backs. The commanders seemed pleased with the exercise and especially happy not to talk about politics.

When asked if he were worried about Mr. Trump’s friendliness with Russia, Sami-Antti Takamaa, a Finnish general, said, “It doesn’t worry me at all.”

“The U.S. airborne division just came from Alaska,” he said. “That’s what matters to me.”