John Avlon is a CNN senior political analyst. The opinions expressed in this commentary are his own. View more opinion articles on CNN.

Children born on that tragic day are old enough to serve and fight in America's longest war.

In New York City, there are 13 members of next graduating class from the Fire Academy who lost their fathers in the destruction of the World Trade Center, according to the New York Post

For many Americans there is an absence of memory about 9/11 -- but for others it persists because of the memory of absence.

Even this year, we've seen first responders have to fight for benefits they were promised by politicians who swore they would never forget, with aid from comedian Jon Stewart, who highlighted what they sacrificed.

And we've seen a president -- from New York -- float the awful idea of hosting the Taliban at Camp David for withdrawal talks within days of the 9/11 anniversary.

This kind of 9/11 amnesia is naive in the extreme.

Terrorism is always one bad day away from being the number one issue in America.

We've been able to forget not simply because of the passage of time, but because of the success of our law enforcement and intelligence services that have stopped so many would-be attacks. These threats have become background noise.

US In the past year alone, we've seen multiple residents convicted of supporting ISIS or scoping out terror targets for Hezbollah.

We also saw a former US infantryman charged with trying to set off a bomb in Long Beach, California, and a Syrian-born resident of Pittsburgh charged with planning an attack on a church in the name of ISIS.

These are just some of the foiled Jihadist plots that we know about.

But in the past year, we've been forced to confront a growing threat from another form of violent extremism: white nationalist terrorism.

We've seen the deadliest attack targeting Latinos in recent US history at a Walmart in El Paso. We've seen an attack on a synagogue near San Diego, which echoed the attack on the Tree of Life synagogue in Pittsburgh, believed to be the largest massacre of Jewish-Americans in our history.

These terror suspects echoed white supremacist conspiracy theories about "replacement," hatred toward immigrants and refugees and ambitions of sparking a race war.

And then there are the would-be attacks thwarted by law enforcement, including an arrest of an Ohio man who police say threatened a Jewish community center and the arrest of a Coast Guard officer , with a massive weapons cache, who prosecutors say was plotting to kill Democrats and journalists.

This is a growing problem. The FBI reports that of domestic terrorism cases with a racial component, most are linked to white supremacy.

And here's a startling statistic: since the 9/11 attacks, right-wing terrorists have killed more people in America than jihadist terrorists, according to the New America think tank

There are some folks who, for their own political purposes, would like to keep the focus on one form of political violence over another.

But that would be unwise.

Because we don't have the luxury of choosing which threats we face.

And there's a strong case to be made that these opposing terrorist threats actually echo each other, as my colleague Jim Sciutto and others have argued.

Terrorism is a weaponized version of tribalism, motivated by fear and finding identity in their hatred of "the other." As Max Fisher of the New York Times wrote, "In both, there is the apocalyptic ideology that predicts -- and promises to hasten -- a civilizational conflict that will consume the world."

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To mark the 18th anniversary of 9/11 is to reflect on how we are all the children of 9/11.

That attack unleashed destructive forces that we are still wrestling with.

But to truly learn the lessons of 9/11 is to resolve not to let hate win or fear define us.

It is to remember that as our first responders ran into the fire on that day, we met the worst of humanity with the best of humanity, and that our greatest strength as a country comes when we refuse to engage in group blame but instead stand united against those who would divide us.