By imposing heavy tariffs on steel, aluminum, and household appliances, President Trump is rejecting the consensus of economists on the benefits of trade. This general view, which dates back to Adam Smith and David Ricardo, is straightforward: If countries import goods that can be made more cheaply abroad than at home, and produce goods that can be made more cheaply at home than abroad, then all trading nations will end up with more income and wealth than if they tried to produce everything themselves.

As powerful as this...