What happens to U.S. politics after Donald Trump? Do we snap back to normal, or do things spin ever more widely out of control?

The best indicator we have so far is the example of Italy since the reign of Silvio Berlusconi. And the main lesson there is that once the norms of acceptable behavior are violated and once the institutions of government are weakened, it is very hard to re-establish them. Instead, you get this cycle of ever more extreme behavior. The political center collapses; the normal left/right political categories cease to apply, and you see the rise of strange new political groups.

Silvio Berlusconi first came to power because voters were disgusted by a governing elite that seemed corrupt and out of touch. They felt swamped by waves of immigrants, frustrated by economic stagnation and disgusted by the cultural values of the cosmopolitan urbanites.

Berlusconi degraded public discourse with his speech, weakened the structures of government with his corruption and offended basic decency.

Berlusconi did nothing to address the sources of public anger, but he did erase any restraints on the way it could be expressed.

This past weekend’s elections in Italy were dominated by parties that took many of Berlusconi’s excesses and turned them up a notch.

Italy is now a poster child for the three big trends that are undermining democracies around the world.

First, the erasure of the informal norms of behavior. As Steven Levitsky and Daniel Ziblatt argue in “How Democracies Die,” democracies depend not just on formal constitutions but also on informal codes.

Second, the loss of faith in the democratic system. Seventy-one percent of Europeans and North Americans born in the 1930s think it’s essential to live in a democracy, but only 29 percent of people born in the 1980s think that. In the U.S., nearly a quarter of millennials think democracy is a bad way to run a country. Nearly half would like a strongman leader. One in 6 Americans of all ages support military rule.

Third, the deterioration of debate caused by social media. At the dawn of the internet, people hoped free communication would lead to an epoch of peace, understanding and democratic communication. Instead, we’re seeing polarization, alternative information universes and the rise of autocracy.

The underlying message is clear. The populist wave is still rising. The younger generations are more radical, on left and right. The rising political tendencies combine lavish spending from the left with racially charged immigrant restrictions from the right.

Nothing is inevitable in life, but liberal democracy clearly ain’t going to automatically fix itself.

— David Brooks is a columnist for The New York Times.