Francis Fukuyama is still bullish on where history is headed, but Americans should worry: republics can decay.

Francis Fukuyama made himself famous overnight in 1989 with the claim—in his essay “The End of History?”—that history as we knew it had ended with the victory of liberal-democratic capitalism over Communism. In fact, his tract wasn’t as triumphalist as some now remember. Fukuyama wondered, with Nietzschean melancholy, whether citizens in the newly hegemonic West would lose spiritual and moral purpose now that the all-defining conflict with Communism was over.

Capitalism did win in 1989—no credible alternative has emerged—but capitalism didn’t lead to liberal democracy. Market systems turned out to be politically promiscuous: they could share a bed with any number of political regimes, from Nordic democracies to Singaporean meritocracies. In Xi Jinping’s China and Vladimir Putin’s Russia, Western liberal democracy now faces a competitor Fukuyama did not anticipate: states that are capitalist in economics, authoritarian in politics, and nationalist in ideology. These new authoritarians are conducting an epoch-making historical experiment as to whether regimes that allow private freedoms can endure when they deny their citizens public freedom.

Fukuyama has now completed the second of two enormous tomes on the history of political development from the dawn of civilization to today. (The Origins of Political Order: From Prehuman Times to the French Revolution came out in 2011.) Far from recanting, he appears to double down on his original claim that history has a democratic destiny. After exhaustively and sometimes laboriously tracing the political development of societies around the globe—Political Order and Political Decay: From the Industrial Revolution to the Globalization of Democracy is 672 pages long—he concludes that, yes, “there is a clear directionality to the process of political development.” Democracy is where political history is headed, he says, and “the prospects for democracy globally remain good.”

This assessment depends greatly on the global rise of the middle class. “No bourgeois, no democracy,” the Harvard theorist Barrington Moore Jr. famously wrote. Fukuyama cites figures showing the worldwide middle class expanding from 1.8 billion people in 2009 to a projected 4.9 billion in 2030. As their incomes rise, he argues, they demand rule of law to protect their property and then demand political participation to safeguard their social standing. They do so not just to defend their economic interests but also for moral reasons. Beyond a certain level of status and income, people become insulted when authoritarian systems of rule treat them as disobedient children.

Fukuyama has learned caution since “The End of History?,” and some readers will tire of his tendency to hedge his bets about the implications of his own theories. If his analysis is true, however, then Presidents Xi and Putin should beware. Over the long term—and nobody knows how long that might be—authoritarian regimes that allow their citizens capitalist freedoms but deny them democratic rights will explode, in revolution, coups, civil war, or a combination of all three. Democratization, Fukuyama seems to be saying, will eventually turn out to be necessary to Russia’s and China’s very survival as unitary states...