Five years ago, then-New York City Mayor Michael Bloomberg visited Newsday’s editorial board. He said something that haunts me as I see how our president and his supporters are behaving.

“It doesn’t have to stay nice, you know.”

Bloomberg was talking about the city and its rebirth since the 1990s. His point was that the city’s politicians and its people had to stay fanatical about maintaining the institutions and initiatives that fed the resurgence.

Now I hear his admonition when I think of the nation. It seems people don’t understand the United States is still a place where all can still go wrong.

We seem to see overwhelming disaster as a thing of the past. A pandemic is the Spanish Influenza of 1919. A depression mans 1929. A war with tens of millions fighting is so 1941.

And a civil war? Our nation torn asunder by political and cultural disagreements? That could never happen here, right? Even though it has?

Our institutions and democracy overcome by a dictator? A military government running the show? These tragedies seem impossible in our colorized world. But they aren’t.

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It’s almost inevitable that we’ll see another pandemic hit at some point, and kill millions. Imagine how many would have died if AIDS had been transmitted via air rather than bodily fluids. It’s almost inevitable that we’ll see another devastating depression. The Great Recession of 2008, had the meltdown of the financial markets gone just a bit further, could have been as bad as 1929.

Someday the San Andreas fault will truly shudder and California will drop. Someday the Yellowstone caldera will blow, an explosion far more powerful than any nuclear weapon.

But civil wars and dictatorships and the breakdown of democracy coming to the United States? That is not inevitable. But it’s possible.

And the sense that it’s impossible, that we can let institutions and principles be attacked because it will be all right, because we are Americans, is terribly dangerous.

Some worry President Donald Trump will serve less than four years. The scarier danger is that he will serve more than eight. And that his supporters will cheer him as he does.

Trump, whose attacks on a free press are constant, played along this week as Philippines President Rodrigo Duterte called journalists spies and said, “I’m very happy to hear you have trouble with the press also.” Hundreds of journalists were killed in the Philippines in the past three decades.

Trump, who consistently attacks U.S. intelligence agencies, has said he believes Russia’s Vladimir Putin over those agencies. Trump undermines the judiciary, claiming judges who thwart him are illegitimate. Trump undermines the Justice Department, demanding it pursue his political opponents.

And his supporters cheer him on, not knowing or caring that they are cheering the destruction of this great nation.

Some GOP leaders have talked about postponing a special election for a U.S. Senate seat in Alabama because their candidate, accused of sexual predation on minors, may not win. Trump himself was accused of sexual predation by numerous women. He promised he’d sue them for defamation right after the election, but that was a lie.

Talk of postponing elections has a whiff of looming fascism. So does letting leaders lie with impunity.

The concept of morality is under attack. The idea of truth, too. Trump’s strongest fans argue that any narrative that goes against their side cannot be true, even if it’s built on the president’s own quotes.

This is how dictatorships start. Our nation does not have to stay nice. Terrible things can happen here, if we let them.

Lane Filler is a member of Newsday’s editorial board.