Financial markets, which have shown increasing insensitivity to tariff threats from the US, will face a test at the Monday open after President Donald Trump declared a 30% rate for the European Union and Mexico effective Aug.

Goods imported from both the European Union and Mexico will face a 30% U.S. tariff rate starting August 1, U.S. President Donald Trump said in letters posted to his social media platform on Saturday.

Canada is among the US’s biggest trading partners, and flows between the two countries are crucial to business in North America.

President Donald Trump’s threat to boost import taxes by 50% on Brazilian goods could drive up the cost of breakfast in the United States. The prices of coffee and orange juice — two staples of the American morning diet — could be severely impacted if there's no agreement by Aug.

The move showed Mr. Trump’s new willingness to use tariffs to settle political scores, regardless of questions of legality, because of their sheer power to cause economic destruction and impose inten...