The European Union on Wednesday released its target list of retaliatory tariffs on American exports worth $3.5 billion if President Trump pushes ahead with his steel and aluminum tariffs. This is how Mr. Trump’s trade irruptions could imperil American exporters and become a destructive spiral. The EU is acting with some restraint—for now—in crafting a narrow list of items on which to impose tariffs, including bourbon, orange juice, corn, ladders and motor boats. None are vital to European industry, but they are politically shrewd in targeting exports from states represented by Republicans on Capitol Hill. The point is to punish...