President Trump announced Thursday he’s imposing new tariffs on imports of foreign steel and aluminum—in a move that sent world stock markets tumbling amid fears of a new trade war. The new tariffs—25 percent on steel and 10 percent on aluminum—will benefit U.S. producers of the metals, while raising prices for companies that manufacture everything from cars to airplanes to high-rise apartments. The announcement prompted fears that other countries will impose retaliatory tariffs while challenging U.S. protectionism at the World Trade Organization.