Donald Trump made the biggest policy blunder of his Presidency Thursday by announcing that next week he’ll impose tariffs of 25% on imported steel and 10% on aluminum. This tax increase will punish American workers, invite retaliation that will harm U.S. exports, divide his political coalition at home, anger allies abroad, and undermine his tax and regulatory reforms. The Dow Jones Industrial Average fell 1.7% on the news, as investors absorbed the self-inflicted folly.

Mr. Trump has spent a year trying to lift the economy...