ABOARD THE U.S.S. IWO JIMA, on the Norwegian Sea — Among hundreds of Marines boarding amphibious assault vehicles this week to get from the frigid sea to the frigid beach, Lance Cpl. Jacob Boutte was armed with a secret weapon: black Merino woolen long johns.

They were not part of the Marine Corps’s standard kit during 17 years of deployments to the warmer climes of Iraq, Syria, Djibouti and southern Afghanistan. So as he prepared to embark this week on one of the North Atlantic Treaty Organization’s largest exercises since the end of the Cold War, Corporal Boutte turned to allies for help.

“I talked to the Norwegians about what they use,” he said.

“We haven’t fought in the cold in a long time,” echoed Sgt. Juan Carlos Banda, a platoon leader with the 24th Marine Expeditionary Unit, which is based at Camp Lejeune in North Carolina.