GRAND CANYON NATIONAL PARK, Ariz. — When lightning sparked a small fire amid the stately ponderosa pines on the remote North Rim of the Grand Canyon last month, national parks officials treated it like a good thing.

Instead of racing to put the fire out immediately, as was the practice for decades, they deferred to the doctrines of modern fire science. The prevailing wisdom says the American West was forged by flames that nourish the soil and naturally reduce the supply of dry fuels.

So officials built containment lines to keep the fire away from people and the park’s historic buildings and then stepped back to let the flames perform their ancient magic.

That strategy worked well — until it didn’t. A week later, the wind suddenly increased and the modest, 120-acre controlled burn explod

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