Facebook users can “unfriend” people they don’t like anymore. Today, unfriending the European Union seems to be U.S. policy. The Trump administration’s “America first” trade tactics risk resulting in “America alone.” Neither Europeans nor Americans would profit from that outcome.

From an economic perspective, U.S. trade policy is increasingly difficult to understand. The administration has decided to impose tariffs on steel and aluminum imported from the EU, Canada and Mexico, along with most other countries. The tariffs will...