Equity markets took a header on Tuesday as global troubles intruded on the story of faster U.S. economic growth. The combination of Italy’s political threat to the euro and Donald Trump’s decision to impose unilateral tariffs on $50 billion in Chinese imports had investors retreating from riskier assets as the Dow fell some 1.6%. Mr. Trump likes to cite a rising stock market as policy affirmation, so perhaps he should heed the reverse signal as a warning.
The U.S. can’t do much about the mess of Italian politics—and neither...