Twenty years. Twenty years since the unbelievable happened. Twenty years since a quartet of terrorist attacks took the lives of 2,977 people in New York City, Washington and a field near Shanksville, Pennsylvania.

What is there to say, 20 years on?

We of course continue to remember and mourn the victims, who got up on the morning of Sept. 11, 2001, expecting to go about a normal day in their normal lives.

We of course stand with their families, who endured the most unimaginable and unexpected of losses.

And we honor those first responders who risked and lost all by going into the burning towers in hopes of helping people escape, and the passengers of United Airlines Flight 93 who fought back and saved lives at the cost of their own.

But this year’s anniversary of the 9/11 attacks, being the 20th, is no ordinary milestone.

After 20 years, there’s an entire generation of Americans among us who not only have no direct memory of the attacks, but weren’t even alive when they took place. For them, there’s the same temporal and emotional distance from 9/11 as those of us born in the early 1960s had from the bombing of Pearl Harbor.

One commonality between those generations is uncertainty.

The U.S might have won the war that followed Pearl Harbor, but it remained far from obvious in 1961 that it would win the peace. The Berlin Wall went up that same year. Vietnam followed. Everyone wondered whether a third world war was coming, just as the second had shortly followed the first. Doubt lingered until the wall came down in 1989 and the Soviet Union fell in 1991.

Twenty years after 9/11, there’s similarly reason to wonder about the ultimate outcome. Yes, bin Laden has been killed, the mastermind of the attacks is in Guantanamo, the World Trade Center has been replaced and the Pentagon has been patched.

On the other side, the Taliban have just regained power in Afghanistan, and however radical they are, they aren’t necessarily the most radical of elements in that neighborhood.

Those latest facts aren’t exactly morale builders. We don’t consider either of them all that worrying, having long been of the view that after dealing with al-Qaeda, there was nothing to win and nothing much worth doing in Afghanistan that required keeping conventional military forces there.

But it’s possible we’re wrong. The thing about history is, it’s filled with 50-50 calls. There are generally sound arguments on both sides of them, and the right course of action is seldom obvious.

Being 50-50 calls, they also generate plenty of public debate and second-guessing. One thing that never accompanies them is certainty. In 1981, neither hawk nor dove, no matter how loudly argumentative about the outcome of the Vietnam War, would have given odds on the Berlin Wall being gone in eight years or the Soviet Union disappearing from the map in 10.

What history does reward is fortitude.

We wish more participants in this country’s political debates would grant that being wrong isn’t the same thing as being incompetent or evil, and in the face of uncertainty, mistakes are inevitable. What matters is learning from them while sticking to your goals. The worst sin is being stubborn.

On Sept. 12, 2001, we wouldn’t have given odds against the possibility of another attack of similar scale, scope and violence occurring here. It’s to the credit of the presidents since — and even more to the credit of the many Americans who have worked to protect this country, at home and abroad — that there hasn’t been a second 9/11.

That’s not to say there haven’t been other challenges, the COVID-19 pandemic emerging in 2020 as the most recent and perhaps most difficult of all. But the same lessons of fortitude, learning and flexibility can see us through.