CAMP VIKING, Norway — In the deep snow of the Arctic mountains, Britain’s Royal Marines are readying for war with Russia. The elite troops are introduced to the wilderness by camping in the snow in temperatures below minus 20C. They finish by jumping through ice holes and shouting their name, rank and number before they can be pulled out of the water. Then they roll in the snow, drink a tot of rum, and toast King Charles III. Britain’s extreme weather training in this area dates to the Cold War,