U.S. futures and global stocks were hit by a selloff early on Monday as markets around the world braced for a potential trade war triggered by President Donald Trump’s decision to impose sweeping tariffs on imports from Canada, Mexico, and China—which warned of “necessary countermeasures.”

Canadian officials have said that the goods have been selected with an eye to maximizing the effect of their retaliation in Republican-controlled states, in a bid to get representatives from those states to weigh in with Mr. Trump and ask him to call off U.S. tariffs and de-escalate.