There is nothing inevitable about the expansion of democracy. Among countries with populations above one million, there were only 11 democracies in 1900, 20 in 1920 and 29 in 1974. Only for the past quarter of a century has democracy been the world’s predominant form of government. By 1993, the number of democracies had exploded to 77—representing, for the first time in history, a majority of countries with at least one million people. By 2006, the number of democracies had ticked up to 86.

But we are now at a precarious moment....