On a warm spring day in Montana's Bitterroot National Forest, a plume of white smoke rises from a ridge and drifts slowly across the valley below. Firefighters working for the Forest Service move carefully through the woods with drip torches, a tool that spews ignited liquid fuel onto the ground. Their job is to set the forest on fire. For these disciples of Smokey Bear, setting fires once would have been an unthinkable act. Now scientists have a better understanding of fire's role in the forest and know that it is vital to a healthy forest.

For nearly a century, however, the Forest Service put out fires that it mistakenly believed were damaging to the forests. The agency extinguished fires—started by both lightning and humans —that had historically burned across the American landscape. During this same period...