Economic uncertainty and prosperity are sworn enemies—when uncertainty reigns, prosperity fades. Uncertainty undermines prosperity by sapping investor and consumer confidence, choking off private investment, and suppressing consumer spending. The depression that followed the 1929 crash and the recession that followed the 2008 financial crisis are called “great” not only because of the magnitude of the downturns, but also because the economic uncertainty that followed produced the weakest recoveries of the past century. Today, the Trump administration’s trade policies have increased economic uncertainty to a level that...