Eighteen years on, the totality of what happened to America on Sept. 11, 2001, remains in some sense difficult to properly assess.

The sheer loss of innocent lives, some of them with close ties to New Hampshire, still stuns.

The bravery of American civilians, willing to die rather than have their passenger jet become another weapon against their country, still inspires.

The selfless work of first responders, who raced INTO the burning World Trade Center twin towers as they began to crumble, echoes to this day.

The sense of unity, patriotism and determination in the days and months following the attacks, and the “how can we help?” attitude was palpable.

We also learned, in hindsight, that we had let our guard down; that, as with Pearl Harbor, 1941, we were not as watchful as we needed to be to protect our nation.

Eighteen years on, elite American warriors have tracked down and eliminated Osama bin Laden, the mastermind behind the attacks that day. But our military remains fighting and dying in Afghanistan and, but for a presidential tweet last Sunday night, the Taliban that aided bin Laden and keeps up the killing would be sitting right now at Camp David as our invited guests.

We still have lessons to learn, including keeping our guard up and continuing to try to find ways to unify in order to remain free.

That is what we owe to those who died that awful morning and what we owe to our country and our children today.