Much of Wall Street had been hoping Trump’s talk of tariffs through the presidential campaign was just that, talk, and an opening point for negotiations with U.S. trading partners instead of a permanent policy. Monday’s swivels on Mexico and Canada leave open the question of whether Trump is using tariffs as merely a tool for negotiations.

But when traders came into Monday morning thinking tariffs were imminent, fear rose quickly about the potential for an escalating trade war that could damage economies worldwide, including the United States.

“Living in the Midwest, I might feel the trade war soonest and most,” said Brian Jacobsen, chief economist at Annex Wealth Management, because of how much crude oil flows over the northern U.S. border to make gasoline. “Our refiners can’t easily switch away from Canadian crude.”

Crude oil prices swung Monday amid the uncertainty. The price for a barrel of benchmark U.S. crude went from $72.53 on Friday to nearly $75 before the U.S. stock market opened Monday to briefly falling back toward $72.

Trump himself warned Americans they may feel “some pain” from the tariffs, which he said would be “worth the price” to make America great again. He also said Sunday night that import taxes will “definitely happen” with the European Union and possibly with the United Kingdom as well.