The president, who has threatened and delayed other tariffs, said he will announce the new measures Monday. Canada and Mexico are among the U.S.’s top steel suppliers.

Mr. Trump’s decision to impose tariffs on metals is not new. In his first term, the president levied tariffs on foreign steel and aluminum globally, angering allies like Mexico, Canada and the European Union. He ultimately rolled back some of those barriers on Canada and Mexico when they signed a revised trade agreement with the United States.

President Trump delayed aggressive tariffs on imports from Canada and Mexico, but people close to him warn that more duties are coming—and won’t so easily be averted.