The sobering news arrived in San Juan via telephone from Washington.

It was April 28, and U.S. Treasury Secretary Jacob J. Lew called to tell Puerto Rico officials they must confront one of the island’s gravest financial crises without a bailout. Saddled with $72 billion in debt, the commonwealth, a U.S. territory since the Spanish-American War, needs a “credible” plan, Lew said.