It was a harsh realization 15 years ago. The realization that this was no accident. Unknown enemies had purposefully commandeered passenger planes and crashed them into the World Trade Center twin towers and the Pentagon. The realization that our nation was under attack. The anxiety of what more was to come. Our collective sense of invulnerability as a nation was left shattered.

The realization, perhaps most profoundly, that something had permanently changed, separating our world before 9/11 and our world after.

The 9/11 attacks ended that brief period when our collective fear had waned. The Cold War was over. Concerns of a civilization-ending nuclear exchange between two great powers with stark ideological differences had, if not disappeared, greatly diminished.

The United States had emerged as the lone super power. Who could threaten us?

As it turned out, a small group of suicidal fanatics armed only with box cutters.

Nearly 3,000 murdered; innocent civilians and heroic rescuers, in New York City, Washington D.C., and a field in Pennsylvania.

Who were the people who had attacked us? Why did they so hate us?

What we saw as our free and tolerant culture, they saw as permissive and evil. These enemies perceived our foreign policy, aimed at extending our values globally, as an extension of colonialism. They were already at war. We didn’t know it.

That war continues, 15 years later.

But we also recall the positives. How the country pulled together. Only 10 months earlier, President George W. Bush had won election despite losing the popular vote. Many Americans considered the victory invalid, stolen when the conservative justices of the U.S. Supreme Court voted 5-4 to end a challenge to the 537-vote Florida victory that gave Bush his Electoral College majority.

All the more remarkable, then, that the country rallied behind its president when, standing on the rubble of the towers and with onlookers shouting they could not hear him, he declared through a bullhorn, "I can hear you! The rest of the world hears you! And the people – and the people who knocked these buildings down will hear all of us soon."

And they did. Under a subsequent president, Special Forces would hunt down and kill Osama Bin Laden, mastermind of the 9/11 attacks. A relentless campaign has disrupted the operations of al-Qaida and killed its leaders. There has never been the massive follow-up attack that Americans feared in the weeks, months and even years after 9/11.

Another memorable moment came six days after the attacks when Bush visited a Washington mosque, stating, “The face of terror is not the true faith of Islam. That’s not what Islam is all about. Islam is peace. These terrorists don’t represent peace. They represent evil and war.”

Bush recognized, as does his successor, President Obama, that painting with a broad stroke and making all Muslims the enemy is not only wrong, but would be a damaging mistake. The forces that attacked the towers and their progeny, the Islamic State, would want nothing more than to paint this as a war of religions and culture, of Islam against the West.

This is why it so dangerous when the Republican presidential nominee, Donald Trump, talks of banning or requiring “extreme vetting” of Muslims who want to enter the country. Or when he talks of his secret plan to defeat the Islamic State, and in the same breath alludes to seizing the oil fields of vanquished enemies.

The Democratic nominee, Hillary Clinton, following in large measure the strategy of the Obama administration, recognizes that assisting the surrounding Islamic nations — with air support, intelligence and Special Operations forces — and allowing them to defeat the blight on Islam that is the Islamic State, places the focus were it must be.

Trump is a terrorist propaganda tool. Fifteen years after the worst foreign attack on domestic soil, Americans cannot risk making him commander-in-chief.