THE DOZEN states of America’s Midwest have a population of 68m, equal to Britain’s. They share an economy worth some $4trn, equivalent to the GDP of Germany, the world’s fourth-biggest. And the region’s swing voters weigh heavily in politics. Donald Trump won the presidency four years ago thanks to narrow victories in Michigan, Pennsylvania and Wisconsin. This year’s contest may yet be decided there, too. At the same time the Midwest’s troubles, after decades of industrial decline, are also outsized. Detroit, despite its recent improvements, is sadly still emblematic of how hard it is for cities to recover. It has shrunk to just one-third of its peak population of 1.8m in the 1950s. What can the region do to prosper again, and what can the rest of the world learn from its experience?

The Midwest as a whole draws in too few migrants, and too many of its brightest decide to leave. Until that is reversed, renewed prosperity is likely only in some urban parts, not everywhere. A sort of triage is under way. Not everywhere can be saved. Sometimes outsiders—state or federal government perhaps—will have to try to make it easier for people to live, work and study elsewhere, for example by cutting housing costs for those who move or helping pay for better education. But even Flint, Michigan, which has become notorious for economic decline and...