In retrospect, the toll of Sept. 11, 2001, was far more than the thousands of souls who perished in the attacks on the Twin Towers, the Pentagon and in Pennsylvania.

It marked the end to a different age in U.S. politics, one in which the central policy questions were domestic. Wars in Afghanistan and Iraq followed, deeply involving the United States in those countries. The Afghan conflict continues today. These sustained efforts have exacted real tolls, both an American lives and those living in the countries.

National politics of always been reactive, but it is difficult to imagine that Barack Obama would have been elected president without his early opposition to the war in Iraq. Likewise, Donald Trump gained notice with his call to bring American troops home from foreign fields.

The point here is not to argue the wisdom of any American military conflict. The point is simply to note that the Sept. 11 attacks unspooled a grand arc of history that we’re still navigating through. Had they not occurred, it’s possible that our country would tackled other challenges, in other ways.

We will never know. But as we peer over the precipice of a world made unrecognizable by climate change, as we grapple with the reality of governments across the globe stretched and tested by populist leadership, it is clear that that terrorism isn’t the sole threat we face. The challenges are vast, and dark, and even with the best possible intentions, we may not be able to truly solve them.

What remains, now, nearly two decades on, is the bravery of those souls who perished and fought to save lives on Sept. 11. What remains is the bravery of the troops who embarked to the Middle East in wave after wave over the past two decades. What remains, perhaps most sadly, is the brief fleeting moment of national unity that emerged in the days after the attack.

There was a sense that America was truly united. Not in jingoistic chest thumping, but in common empathy and common pride.

That unity may have all but disappeared, but today, rather than remembering the horror of the attacks, let’s remember the sense that all of us, together, can overcome the obstacles placed in front of us. That’s the feeling that we truly need today, as we face a daunting future.