President Trump’s tariffs on steel and aluminum imports are mostly pure protectionism. Canada, Brazil and Mexico are the main suppliers, not the state-driven over-production of China. But now the President says he is going to impose reciprocal tariffs, matching dollar for dollar (percent for percent?) what other countries impose on American products. That has the air of equity, a term Mr. Trump does not like in other contexts.   The fact that some countries have higher tariffs and others have lower tariffs is a vestige of larger historical differences between different economies. Former USTR official Mark Linscott blames Trump’s move toward reciprocal tariffs on the 30-year failure of the WTO to get its members to volu...