As dawn broke on Sept. 11, 2001, America was asleep -- believing itself to be prosperous, safe and secure. Hours later, we gazed in horror and disbelief as the Twin Towers collapsed in a maelstrom of flame, smoke and debris.

What did we learn? First and foremost, that there is evil -- real evil -- in the world.

Evil regularly resurfaces in history. A generation ago, it took the form of the Soviet menace, and before that, of Hitler and his gas chambers. On Sept. 11, it crashed in on us in New York City, where it destroyed our complacent assumptions about the sort of the life we could take for granted as 21st-century Americans.

This lesson caught us unawares. For decades, our opinion elites had peddled a facile moral and cultural relativism that denied the stark reality of evil. This relativism insisted that all ways of life -- all modes of thought -- are equally valid, and that "tolerance" is the only real virtue. It counseled that when faced with malevolent adversaries, we should express good will, seek to clear up misunderstandings, and consider changing our own behavior in hopes of placating our enemies.

Relativism resurfaced in certain quarters in the months after the World Trade Center's destruction. While the ashes were still smoldering, some members of the media elite, academics and others in the "chattering classes" began suggesting that America was somehow to blame for the terrorist attacks, and to counsel tolerance and understanding.

But Sept. 11 convinced most Americans that evil is real, and that we must defend against it. That day brought a dawning awareness that our nation -- so strong, so free, so prosperous -- is in fact fragile. We came to see that the blessings we experience here are not inevitable, and that we dare not take them for granted.

This sense of vulnerability grew as new horrors followed Al-Qaida's attacks. Sept. 11 demonstrated that terrorists could cripple our democracy by crashing a plane into the U.S. Capitol and wiping out our elected representatives, as Osama bin Laden's henchmen apparently planned to do. Not long after the attacks, letters laced with deadly anthrax began circulating through the U.S. mail, addressed to congressional offices and news organizations. At least 22 Americans were infected, and five died.

The threat of biological warfare -- once the stuff of sci-fi movies -- became alarmingly real. We learned that anthrax spores scattered by a small plane could annihilate hundreds of thousands of people, while smallpox could kill millions. The government scrambled to stockpile smallpox vaccines, and we contemplated what it would be like to die from nerve gas, ebola or bubonic plague.

Then a thwarted Al-Qaida plot revealed a new danger: "Dirty bombs" that could transform downtown Chicago or Seattle into a radioactive wasteland, spreading fear and panic. Was any place safe? Could we ever again go to the Mall of America or a Vikings game without glancing around for "suspicious activity" and risking being blown to bits?

Yet while Sept. 11 highlighted the dangers we face, it also revealed the great reservoir of strength, resilience and virtue in the American people. On that day and many thereafter, we witnessed heroism of a sort we didn't know was still possible.

New York City's valiant firefighters and police performed with exceptional courage and resolve, and hundreds of them lost their lives in the line of duty. Ordinary New Yorkers hastened to each other's rescue, and the city's construction workers worked tirelessly -- and risked health hazards -- to clear the rubble and rebuild.

Most uplifting, perhaps, was the sense of unity that galvanized Americans of all backgrounds throughout those dark days. In an age of identity politics and group grievances, it was inspirational to see that -- for black and white, male and female, rich and poor -- our shared identity as American transcended everything that divides us.

In 2001, we would have been astounded to learn that America would be spared another Sept. 11 -- or anything remotely like it -- in the ensuing decade. For this, we must thank national leaders who were willing to take the fight against terrorism to the enemy, and to use aggressive counterintelligence measures despite ferocious vilification from the left.

Today, our challenge is to remain vigilant, and to keep before us the lessons of the last 10 years. Among them are the reality of evil, the emptiness of moral and cultural relativism, the fragility and vulnerability of our way of life, and our duty to pass on the principles -- and the courage, resolve and public-spiritedness -- that helped this nation climb out of the ashes of Sept. 11, 2001.

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Katherine Kersten is a senior fellow at the Center of the American Experiment. The views expressed here are her own. She is at kakersten@gmail.com.