Free trade, for all its virtues, isn’t an easy sell. Every country applies some level of protectionism out of economic or political necessity.

But protectionism can be done well or done badly. Done well, it minimizes costs to taxpayers and consumers, maximizes benefits to domestic industry, and discourages bad behavior by trade partners. President Donald Trump routinely does protectionism badly, using the wrong tools on the wrong behavior and the wrong countries.

On...