On Tuesday, President Trump told reporters he plans to raise tariffs on copper to 50 percent. Copper futures surged by 17 percent shortly afterwards—the largest intraday gain since at least 1988. Many will make—or lose—a small fortune on Wall Steet in the coming days and weeks. No doubt, the winners will gas-up the Presidency for protecting America’s economy: tariffs are good! Meanwhile, the losers will accuse him of economic illiteracy: tariffs are bad! Which is it? Are tariffs good or bad? The answer: both. Tariffs are good when they promote economic growth or economic self-sufficiency. If not, then tariffs are...