Contrary to President Trump’s latest boast, trade wars are not good and not easy to win. U.S. agriculture now feels threatened by the effort to protect the American steel and aluminum sector. What is notable, though, is the similarity between food and steel. Production of both is deemed in the national interest; thus, they find state support that eventually results in excess production, and it becomes difficult to unwind the resulting distorted markets. More advanced steelmaking employs alloys like manganese, nickel and chromium, but the fundamentals are basically iron ore plus carbon and the energy to transform them. Japan is the number three steel producer in the world, and yet it has very limited domestic sources of either ore or...