President Trump sure knows how to spoil an economic mood. Three days after he signed the GOP’s big budget bill, saving the economy from a scheduled $4.5 trillion tax increase, Mr. Trump was back playing the role of Tariff Man. On Monday he announced 25% tariffs on Japan and South Korea, while adding to renewed will-he-or-won’t-he uncertainty for the U.S. economy and trading partners. In letters to Japan’s Prime Minister and South Korea’s President, Mr. Trump huffs and puffs again about bilateral trade deficits, which he mistakenly thinks are a sign of foreign exploitation. “We must move away from these...