HAVANA, Aug. 6 -- Late on Sunday night, the lights went out again across Cuba. Not in one neighborhood, not in one city, but everywhere, all at once, for the fourth time this year. Union Electrica de Cuba, the state-run grid operator, announced the collapse on its official social media accounts without specifying which generator had failed or in which province the cascade had begun. What it did not need to explain is that this is what a sustained fuel shortage looks like after seven months of accumulation.

The collapse on Sunday follows three nationwide blackouts in July alone. Approximately 10 million people, the entire population of the island, were left without electricity as an aging grid operating on depleted reserves gave out under...