Today’s presidential candidates are playing recklessly with free trade, alliances and immigration. They are pushing the misguided notion that high trade barriers will restore jobs and prosperity to the middle class, and scorning old alliances and new immigrants. These protectionist and nativist ideas aren’t new; they’re as old as the Greeks. Athens tried them but it created international disorder and the opposite of the desired result.

While nationalism will always be fodder for politicians, today’s leaders need to understand...