The firms invested billions of dollars on Mexican factories that make products for the American market, shipping goods tariff-free under a U.S. trade agreement now in peril.

The initial justification for the off-again, on-again tariffs on Canada and Mexico — as well as tariffs on China, which are in effect — cited what the White House said was these countries’ failure at “stopping poisonous fentanyl and other drugs from flowing into our country.