When Donald Trump makes a national-security argument for raising the cost of imports of metal, as he did last week, his base may think he’s getting tough with dictatorships like China. But the 25% tariffs on steel and 10% on aluminum that he announced Thursday will hit Canada hardest.

Canada is the largest exporter of steel to the U.S.; China ranks 11th. In 2016 Canada exported 5.4 million metric tons to the U.S. and bought more than 50% of U.S. steel exports. Canada is also America’s largest supplier of aluminum. In 2016...