The horror of Sept. 11, 2001, is beginning to recede into memory, shrinking in the rearview mirror of history with the passing of other terrorist attacks on U.S. soil — Fort Hood, the Boston Marathon, Orlando — and countless acts of jihadist bloodshed around the world.

Nothing has shattered our sense of security as much the crashing of two jets into Manhattan’s Twin Towers, mighty skyscrapers that burst into flames and, incredibly, collapsed to the ground. Another jet crashed into the Pentagon. A fourth went down in a Pennsylvania field, deflected from its Washington target by courageous passengers.

On Sept. 12, 2001, we awoke as a different nation — horrified, yet united by grief and determination.

Fifteen years later, we are a different nation again — divided and callous — and we live in a different world.

Nearly 3,000 people lost their lives in the 9/11 attacks. Our duty as survivors is to remember the souls killed by the planes’ evil trajectory, the brave men and women who ran to their rescue, and the devastated families they all left behind — and to be sure that our nation is not caught so vulnerable again.

More than $1 trillion has been spent on domestic counterterrorism efforts since that day. There’s no way to know how many attacks have been thwarted or discouraged by our ramped-up security, no way to tell which dollars were well-spent. We only know that vigilance must be constant.

There’s one more way to honor the fallen of 9/11 — by being honorable people ourselves and giving up the divisiveness that diminishes our great country. The United States is a strong, wealthy nation, still the envy of the world. We get carried away with internal squabbles, and are so focused on partisan power plays that Congress can’t even agree on funding to fight the Zika virus. The tenor of the presidential campaign is an international embarrassment.

This, too, will pass, but the losses of 9/11 are permanent. On a bright, sunny day 15 years ago, thousands of innocent people met an unthinkable end. In New York, Washington and Pennsylvania, they were attacked simply for being Americans. We must never forget the price they paid and never let those who hate our way of life prevail in their quest to end it.