I was a sophomore in college when the planes hit. My roommates and I sat transfixed on a futon, glued to the horror unfolding on NBC’s Today Show. Years later, I would find myself carrying an M-4 in Kabul, Afghanistan – the country that had hosted the men who had conceived and implemented unimaginable attacks against other human beings. But in that moment I just sat there, stunned.

Hundreds of miles away from my dorm room, thousands of New Yorkers from all walks of life came together to respond to the urgency of an unthinkable act and a crumbling skyline. Americans did the same in the halls of the Pentagon and in a field near Shanksville, Pennsylvania.

We all have personal stories that we share on days like today. Where was I on that September morning 18 years ago? We tell and retell where we were because that was where my generation lost its innocence after the relative peace of post-Cold War America, suddenly witnessing how a terror network – hosted by a rogue regime presiding over a failed state – could bring the most powerful country in the world to a halt.

In those early days, we set aside our differences and stepped towards somewhere kinder. America felt more decent in its mourning, more morally aware. We were reminded of the energies we can summon during moments of crisis. Around Ground Zero, we discovered something approaching common ground.

But as quickly as we found that unity, it slipped from our grasp. Common ground gave way to fear and division, to knee-jerk infringements on our freedom and a self-defeating invasion of Iraq.

Today, the urgency of our era is not captured in a single defining moment. We are faced not with great towers falling one awful morning but with seas rising over the course of decades. Threats from religious fanatics plotting in caves give way to the violence of those hidden among us willing to tear apart a mall in El Paso, a synagogue in Pittsburgh, a high school in Florida, a concert in Las Vegas, a nightclub in Orlando. The challenge of our time is to meet these cumulative dangers with the maturity, unity, and determination with which we met those first awful hours on that crisp September day 18 years ago.

We must respond with sacrifice, honoring the first responders who rushed towards the trembling towers for the sake of other lives and at risk to their own.

We must respond with the respect we showed the fallen regardless of race or religion, finding common ground in our common grief.

We must respond with service, like the thousands of service members who enlisted to protect our freedoms at home and abroad. And we must respond by keeping our promises to those who have served and to the families who have supported them. Eighteen years after those towers came down, I fear that we may one day receive news of the first U.S. casualty of the 9/11 Wars who was born after 9/11. We owe it to them and to their families to bring an end to endless war and to ensure that those who have borne the battle are properly cared for and welcomed back into their communities when they return.

Most of all, we must respond by locating once again that day’s fleeting feeling of togetherness. And if we find it, our greatest challenge – and our greatest accomplishment – will be to sustain it. Events that reshape our lives also bring out our shared humanity. If we can restore Lower Manhattan and the Pentagon, we can restore the common foundations that unite us as Americans. On this day of remembrance, there can be no more worthy tribute than that to those we have lost – at Ground Zero, on the battlefields of Iraq and Afghanistan, in the school rooms, shops, churches and synagogues that have become unspeakable killing fields here at home.

On one of its worst days, America found its best self. And on this day, it is within us to find it once more.

Pete