No one saw it coming—that the next big thing of the 21st century would be the nation-state, an idea from the 17th. Yet it has suddenly become a global phenomenon—a driving force of politics in the U.S. and around the world and the subject of intense intellectual debate. The news has even come to Harvard, where a professor of history has written a book about American nationhood, and a professor of economics says that “there is something special about the nation-state—it creates reciprocal obligations that don’t exist across national borders.”

There...