When President Trump signed his new Nafta accord last month with the leaders of Mexico and Canada, Prime Minister Justin Trudeau almost didn’t show up. The reason: Mr. Trump still hasn’t lifted the steel and aluminum tariffs as he promised he’d do if America’s two neighbors signed a revised trade deal.

A month later they’re still waiting. The delay is damaging the U.S. economy and America’s credibility as a trading partner. Though Mr. Trump likes to use tariff threats as a negotiating tactic, his Administration also promised...