When President-elect Donald J. Trump offered Rick Perry the job of energy secretary five weeks ago, Mr. Perry gladly accepted, believing he was taking on a role as a global ambassador for the American oil and gas industry that he had long championed in his home state.

In the days after, Mr. Perry, the former Texas governor, discovered that he would be no such thing — that in fact, if confirmed by the Senate, he would become the steward of a vast national security complex he knew almost nothing about, caring for the most fearsome weapons on the planet, the United States’ nuclear arsenal.