The western world has seen plenty of division and conflict before. But in the past the U.S. was the stabilizer, seeking to align its interests and those of its partners. Today, it is the destabilizer, driven by a belief that its interests and the world’s are at odds.

The relatively buoyant stock market and economy might suggest this doesn’t matter. Tariffs announced to date are tiny relative to gross domestic product and quantitatively matter much less than tax cuts. Even the most committed free traders admit protectionism...