Skjold, Norway—The bullets were real. But fortunately for the Norwegian snipers, this was just an exercise. If it had been real combat, the enemy would have easily spotted the troops in the vast whiteness of the Arctic. The white camouflage uniforms didn’t match the ever-changing color of snow. The men’s breath and shadows were easily seen in the frost. And the extreme cold impaired the sharpshooters’ accuracy by influencing bullet speeds.

It’s tough to wage war in the Arctic.

As melting glaciers open up access to shipping routes and energy troves, countries with polar real estate are modernizing armies just in case. Severe cold is the last frontier in warfare, barring outer space, and the militaries of the United States, Canada, Denmark, Russia, and Norway are finding that the environment presents dangers as challenging as any enemy. When the mercury drops to 55 below Fahrenheit, ballistics malfunction, helicopters can’t take...

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