By most measures the U.S. economy is in excellent health. Yet stocks are sinking, yields on corporate bonds are rising and commodity prices are tumbling—all typical precursors of a slowdown or recession.

The dichotomy is rooted in two unusual features of the world today. First, while the U.S. is surfing a wave of fiscal stimulus, growth in the rest of the world is slowing. That’s undercutting prospects for companies that do business abroad. Second, the Federal Reserve is steadily withdrawing the unprecedented monetary stimulus...