This year on September 11th, Americans will again show solemn remembrance in New York, Washington, and Pennsylvania at sites that are hallowed ground, consecrated by the loss of thousands of innocent souls and the bravery of those who tried to save their lives.

We will never forget them.

But fifteen years after that fateful day, there is something we are starting to forget:  what was learned from the attacks.

Although we vowed never to let it happen again, we are now witnessing the viral spread of Islamist terrorism around the world, and our enemies are bringing the war back to our doorsteps.

Although we vowed never to let it happen again, we are now witnessing the viral spread of Islamist terrorism around the world, and our enemies are bringing the war back to our doorsteps.

Make no mistake—we’ve come a long way since 2001. Our first responders are better prepared.  Our intelligence professionals are connecting the dots.  And our military and law enforcement agencies have made counterterrorism their highest priority.

However, terrorists have come a long way, too.  Gone are the days of Usama bin Laden, who relied on couriers and caves to hatch plots. Today in an age of franchised terror, jihadists are recruiting online and across borders at lightning speed, and we’re not keeping up.

Radicalism is on the rise partly because we lost sight of the key lessons identified after 9/11.

First, we learned that you must define the enemy to defeat it.  

The 9/11 Commission put it best: “[T]he enemy is not just ‘terrorism,’ some generic evil.  This vagueness blurs the strategy.  The catastrophic threat at this moment in history is more specific.  It is the threat posed by Islamist terrorism…”

Sadly, many in Washington still refuse call the threat what it is, and parts of our government have become blind to the root of this evil.

The truth is that we are engaged in a generational, ideological struggle.  Islamist terrorists have perverted a major religion into a hateful worldview, and while their beliefs are not shared by most Muslims, their influence is spreading like wildfire.

The lack of moral clarity has also left us without a coherent plan to counter terrorist recruitment in our own communities.

Second, we learned that in order to stop attacks you must take the fight to the enemy as soon as they rear their head.

Unfortunately, from the start the administration’s counterterrorism strategy was too narrow.  It focused almost exclusively on Al Qaeda, and after claiming the group was “on the run,” President Obama effectively declared the global war on terror to be over.

As a result, emerging groups like ISIS were ignored for far too long, morphing from terrorist cells into jihadist armies before we responded.

We’ve been reminded that delay can be deadly and that this is not a struggle against a single group, but rather a global movement seeking to undermine our way of life.

Third, we learned that if you don’t keep terrorists behind bars, they’ll go back to war against you.  

In fact, we built a prison and created military commissions after 9/11 for exactly this purpose—to keep hardened terrorists where they belong.

Yet the current administration remains dead set on closing Guantanamo Bay, despite intelligence that at least one-third of released detainees have already returned to the fight or are suspected of having done so.

Some of these individuals have since killed Americans, while others continue to target the United States and our allies.

Fourth and finally, we learned that you cannot let power vacuums become terrorist safe havens.

Al Qaeda plotted from Afghanistan with impunity, which is why the 9/11 Commission’s top recommendation was to use all elements of national power to deny terrorists sanctuary.

Once again, though, we have lost our way. In recent years, the White House has stood on the sidelines as radical Islamists fanned out across the Middle East and North Africa, allowing new terrorist hotbeds to emerge and old ones to be reestablished.

Now as a consequence we are seeing an unprecedented pace of terror.

But even though our adversaries have evolved, the battle-tested principles we embraced after 9/11 are still relevant:

- We must be clear-eyed about the danger if we are going to overcome it, much as we were with communism and fascism.  

- We must go after Islamist terrorists wherever they emerge and regardless of their brand name.

- We must focus on capturing terrorists instead of giving them one-way tickets back to the battlefield.

- And we must make it a top national security priority to prevent the emergence of safe havens so they cannot become incubators for a new generation of terrorists.

Fifteen years ago, I watched the television with my five-year-old daughter as the second of the Twin Towers was hit, realizing that it was not an accident but an act of war against our country.

The world has changed, and today my daughter is 20.  But we cannot let the passage of time dull our memories.  Those who don’t learn history’s lessons are doomed to repeat them.

That’s why this month I will release a national counterterrorism strategy to highlight the principles we must stick to in the fight.  We cannot afford for our next president or future administrations to forget them.

To honor the memory of those we lost, it is time to get back to basics and rededicate ourselves to victory in this long war.

Republican Michael McCaul, represents Texas' 10th Congressional District in the U.S. House of Representatives. He serves as chairman of the House Committee on Homeland Security.