As Americans prepare to express their appreciation for the blessings of liberty this week, there is also cause to appreciate rising prosperity in the U.S. Yet many economists wonder how long the good times can last.

By virtually all estimates the U.S. economy just completed an outstanding second quarter with robust growth. Yet despite surging corporate profits, stocks have moved sideways this year after the huge Trump rally that began on Election Night in 2016. Investors are lately hoping that this year’s Trump tariffs are...