“Build them here!” President Trump enjoined European car manufacturers in a June 22 tweet. This seems to be his real goal, assuming it is not confusedly autarky. In a July 1 Fox News interview, he repeated his hope that tariffs on European cars would bring European manufacturers to "build their cars in America." This old mercantilist idea is intriguing. Inspired by that, I tweeted: Why should Americans work to produce stuff for foreigners instead of for Americans? If you can...