Each year around the Fourth of July, in a vale in the Rocky Mountains, a scene from another century plays out. Dozens of rugged-looking men mill around an encampment. They tether their horses and mules to trees. They wear animal skins. And as they roast slabs of buffalo meat over a fire sparked with flint and steel, they share tips on how best to trap beavers and load a flintlock rifle. Who are these guys?

They’re American mountain men—reenactors of the fur trade that flourished in North America from roughly 1800 to 1840. Like the better known reenactors of the Civil War, they’re dentists or lawyers or mailmen in real life. But for a week each year they shake off