Maybe it will start with a failed initial public offering, followed by the revelation of widespread fraud in Silicon Valley. Perhaps energy prices will spike, sapping the finances of anyone who drives a car to work. Maybe a foreign crisis will cause a credit crunch, or President Trump will spark a global trade war. A recession might seem like a distant concern, with the latest data showing that the current, extraordinarily long expansion just keeps humming along. But one will hit eventually, for some reason or another—that’s how economies work. And when it does, the country won’t be ready. The average middle-class household has largely recovered from the Great Recession, which began nearly 10 years ago, in December 2007. The growing economy has started to boost earnings across the income spectrum , and higher housing prices have done the same for net worth. The amount of debt that households owe is falling , too. Yet millions of people remain in perilous financial shape, with little ...