The Trump administration said Thursday it will impose tariffs on steel and aluminum imports from Europe, Mexico and Canada after failing to win concessions from the American allies. Europe and Mexico pledged to retaliate quickly, and financial markets fell amid fears of a trade war. Commerce Secretary Wilbur Ross said the tariffs would be 25 percent on steel and 10 percent on aluminum, and go into effect on Friday, as the administration followed through on the penalties after earlier granting exemptions to buy time for negotiations. President Donald Trump had announced the tariffs in March, citing national security concerns. The European Commission’s president, Jean-Claude Juncker, said Trump’s decision amounted to trade protectionism and that Europe would respond with countermeasures. “This is protectionism, pure and simple,” Juncker said.