Can Spain Sustain?

A generation ago, San Sebastián de los Reyes was a sleepy bedroom community of 27,000 people at the northern tip of greater Madrid. The high point of the local calendar was the traditional festival of Cristo de los Remedios in late August, when residents celebrated with fireworks, bullfights and bull running. The rest of the year, the demands of the town's residents were relatively simple: clean streets, regular garbage pickup and an early bus down Burgos Road to Madrid, where the jobs were.

These days, San Sebastián de los Reyes refers to itself as Sanse — "a way...