If you are dismayed by Trumpism, don’t kid yourself that it will fade away if Donald Trump fails to win the Republican nomination. Trumpism is an expression of the legitimate anger that many Americans feel about the course that the country has taken, and its appearance was predictable. It is the endgame of a process that has been going on for a half-century: America’s divestment of its historic national identity.

For the eminent political scientist Samuel Huntington, writing in his last book, “Who Are We?” (2004), two components...