According to a much-discussed TikTok trend of a couple of years ago, men think about the Roman Empire every day. One reason might be that the fall of Rome is a memento mori — a reminder that we, too, are mortal. No matter how safe, rich and powerful the United States seems, it might all come tumbling down, as things did that day in 476 A.D. when the last Western Roman emperor was deposed.

The American founders consciously modeled their new nation and much of its architecture on Athens and Rome. Ever since, Americans have periodically feared that we are living in the decline-and-fall phase of those empires’ stories.

Today, a mix of factors — from geopolitical tensions and surging debts to the Trump administration’s challenge to free trade and the rule of law — has brought that fear back with force. People are asking whether time is running out for the American experiment, and whether Chinese leader Xi Jinping might be right in declaring that “the East is rising while the West is declining.”

Next year marks America’s 250th birthday. Few golden ages have lasted that long — and unless Americans adopt a new outlook, ours could soon be over, too.

It is always hard to disentangle the causes of cultural decline. In 1984, a German historian compiled 210 explanations historians had suggested for Rome’s fall, from lead poisoning and barbarian invasions to Christianity, moral decline and gout.

After studying dynamic civilizations such as Athens, Rome, Abbasid Baghdad, Song China, Renaissance Italy and the Dutch Republic, I can attest that there is no single explanation. Each golden age had its own character and its own downfall. Often the familiar horsemen of the apocalypse — war, plagues and natural disasters — played a role. But these calamities appeared many times, and civilizations usually bounced back. Cities can be rebuilt, and knowledge is rarely lost.

What proved fatal was something subtler: In the midst of some of these crises, cultures began to lose confidence. Their mentality and intellectual atmosphere shifted.

The ancient Greek historian Thucydides spoke of two mindsets — the Athenian’s, eager to venture out into the world to acquire something new, and the Spartan’s, intent on staying home to guard what he already had.

Broadly speaking, the Athenian spirit is associated with golden ages. When civilizations were open to influences from merchants and migrants, and when they let people experiment with new ideas and innovations, they prospered. This required tolerance of pluralism and surprise, as well as institutions and norms to restrain rulers’ arbitrary use of power.

Indeed, modern China offers another example of the importance of such a mentality. Its rise has been Athenian in character, beginning with Deng Xiaoping’s 1978 “reform and opening up,” when the country embraced entrepreneurship and trade. Xi’s recent Spartan crackdown on freedoms and private enterprise threatens to undo those gains. Productivity has slowed, debt has soared and confidence has ebbed. It’s hard to see the East rising much further that way.

It is difficult to uphold open societies for long. When cultures turned anxious, curiosity gave way to control and open trade to barriers. The populace tended to long for strongmen and hunt for scapegoats. In times of trouble, the Abbasid Caliphate and Renaissance Italy imposed orthodoxy and persecuted heretics. Even open-minded Athens sentenced Socrates to death. In 1672, the usually tolerant Dutch lynched Johan de Witt, the statesman who had led them to prosperity, and purged their universities of Enlightenment thinkers.

The parallels with our world are unsettling. Since the turn of the millennium, the dominant Western mentality has shifted from Athenian to Spartan. We have endured terrorism, wars, a pandemic and economic turmoil, while social media has intensified polarization and politicians have learned how to divide and conquer.

The world looks increasingly dangerous, and the result has been a backlash against trade and migration. That threatens to cut us off from the world’s talents and technologies, recalling the Chinese Ming dynasty’s 15th-century ban on international trade. They sought stability; they got stagnation.

At the same time, two reactive forces to modern pluralism have developed: a hard nationalist right and a radical illiberal left. They present themselves as opposites but are united in their obsession with identity politics and a dream of sameness, in which alternative ideas and cultures are seen as threats. For years, the illiberal left tried to cancel dissenting voices on campuses and in academic curriculums. Now the Trump movement seeks to impose its own orthodoxy, threatening universities, law firms and media companies with government power.

This ambition to enforce one idea on everyone is always presented as a call for unity. In practice, it creates a zero-sum game that fuels conflict. The assassinations of conservative activist Charlie Kirk and Democratic Minnesota state representative Melissa Hortman, the Jan. 6, 2021, attack on the U.S. Capitol and the repeated attempts on President Donald Trump’s life show how quickly angry tribalism can descend into violence.

A historian easily recognizes ominous signs. But we have seen such signs many times before — and have sometimes managed to pull back, restoring social peace, dynamism and hope. The outcome depends on whether those who know better speak up or stay silent, and whether the exhausted majority retreats into internal exile or decides to wrest back control from the extremes.

Great civilizations, history shows, do not end by fate or old age. They end — or revive — by choice, as when Song China lost its entire northern half to invaders in 1127, yet built a new capital, doubled down on openness and innovation, and became more prosperous than ever. Or when Europe, after the devastation of the 20th century, chose peaceful exchange and common institutions over conflict.

The U.S. retains unique strengths. It has independent courts and mostly freewheeling media. It enjoys relative geopolitical safety. As the country approaches its 250th anniversary, we should remember Abraham Lincoln’s remark that no external enemy could by force take a drink from the Ohio River. “If destruction be our lot,” he said, ” we must ourselves be its author and finisher. As a nation of freemen, we must live through all time, or die by suicide.”

Johan Norberg is a senior fellow at the Cato Institute and author of “Peak Human: What We Can Learn from the Rise and Fall of Golden Ages.”