For 20 years since Sept. 11, 2001, U.S. presidents have been saying their anti-terrorism policies have worked, as evidenced by no new attacks on America. While we should be grateful another attack hasn’t occurred, past performance is no guarantee of future success. Fanatics are nothing but patient, as we have seen in Afghanistan.

Joint Chiefs of Staff Chairman Mark Milley has said the U.S. withdrawal from Afghanistan could lead to civil war and the possible “reconstitution of al Qaeda.” That should come as no surprise as the goals of our enemies in the Middle East have been expressed openly for decades. Their actions have proven a commitment to killing Americans and destabilizing democracies and their economies.

Underestimating one’s enemy is always a fatal error as we saw on 9/11. President Obama once dismissed al Qaeda as the “JV team.” In fact, they are the varsity. It is difficult to deter or destroy an ideology whose adherents are willing to die for its cause, especially when they believe they are carrying out orders from their god.

In its March 2015 issue, The Atlantic noted a New York Times story about confidential comments made by Major General Michael K. Nagata, at the time the Special Operations commander for the U.S. in the Middle East. Gen. Nagata reportedly admitted he had yet to figure out the appeal of one of many terrorist organizations. About the Islamic State, he said, “We have not defeated the idea. We do not even understand the idea.” Whose fault is that?

With their win in Afghanistan, terrorists by whatever name can only be further motivated to conduct new attacks. What might those look like? From experience we know they prefer the big event, such as crashing airliners into buildings and blowing up high value targets. There are other options when big targets are not vulnerable.

Lone suicide bomber incidents can also strike fear into a nation, especially if they are sustained and coordinated.

In 2003, Tom Clancy wrote a novel called “The Teeth of the Tiger” about terrorists who sneak across the US.-Mexico border and in coordinated attacks shoot up suburban shopping malls. Fear of terrorists slipping over the border is not fiction, but of growing concern.

Small-scale attacks, undetectable in advance, have been occurring since 9/11.

Two recent incidents demonstrate what we’re up against.

On the Sunday before the last American military planes left Kabul, a gunman shot and killed a Lyft driver in Garland, Texas. He then drove to a nearby police station and began shooting at people inside. The gunman was identified as Imran Ali Rasheed. Police killed him.

The FBI said Rasheed “may have been inspired by a foreign terrorist organization to commit these crimes.” Ya think? Suspected terrorists are crossing our southern border at “unprecedented level,” according to the outgoing U.S. Border Patrol chief. In New Zealand, a man named Ahamed Aathill Mohamed Samsudeen stabbed seven people inside a store where he had bought a knife. Police shot and killed him. Authorities drew the obvious conclusion, calling it a “terrorist attack.”

Who knows how many — if any — terrorists are among the thousands of Afghan refugees now pouring into America? The Biden administration claims the vetting system should weed out anyone who means us harm, but there are no guarantees and the radicals can be expected to lie and perhaps even have forged papers.

After turning back German General Erwin Rommel’s forces in what Winston Churchill called “The Battle of Egypt,” Churchill famously said: “Now this is not the end. It is not even the beginning of the end. But it is, perhaps, the end of the beginning.”

Given the commitment and resourcefulness of this modern enemy, I’m not sure we are even at the end of the beginning.

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