“I smelled smoke, I turned around, and there it was,” Mr. Kile said in an interview on the porch of his barn-red, two-story home this week. Mr. Kile, who has not talked publicly about the fire before, referred to it as if it had been an apparition. “There was nothing I could do,” he said.

During a morning of chores on his ranch three hours north of San Francisco, he had spotted an underground wasp nest. He grabbed a metal stake and pounded it into the hole to try to seal it off, according to an investigation by California’s fire agency. He told investigators he was allergic to stings and wanted to plug the hole.

He was judged responsible for the fire, but not negligent. Under the heading of violations, the report says “Not Applicable.”

The ignition of the so-called Ranch Fire is a testament to the extreme fragility and volatility of the American West, fire experts say. California in particular is such a tinderbox that something as seemingly innocuous as hammering a stake into the ground can unleash an uncontrollable inferno.

Thousands of firefighters worked for more than a month to extinguish the Ranch Fire, which destroyed more than 150 homes and cost tens of millions of dollars to suppress. The fire burned 410,203 acres of California wild lands, an area half the size of Rhode Island, and killed a firefighter who was struck by a falling tree. Although it was one of the largest fires, it was far from the deadliest. The fire in Paradise, Calif., in fall 2018 killed more than 80 people.