Not since Herbert Hoover signed the Smoot-Hawley Tariff has a president chosen to disregard a larger body of informed opinion than President Trump did when he instituted his protectionist trade policy. Based on a series of verifiably false grievances—wages haven’t grown in 50 years, manufacturing has been hollowed out by imports, countries with trade surpluses are “ripping us off”—Mr. Trump used constitutionally questionable powers to abrogate congressionally approved trade agreements and undermine the world’s trading system. Markets convulsed in anticipation of the massive wealth annihilation that would accompany the shredding of global supply chains and a transition to a more...