President Donald Trump on Saturday threatened duties of 30% on products from Mexico and the European Union, two of America’s biggest trading partners, in an ongoing tariff campaign that’s upended global trade since he retook office in January.

In a letter to Mexico’s president, Claudia Sheinbaum, Mr. Trump said that Mexico was not doing enough to curb the flow of fentanyl into the United States and cited that as the reason for the tariffs. Mr. Trump added that Mexican companies were welcome to manufacture their products in the United States to avoid the tariffs.