The innumerable hard lessons learned following the terrorist attacks of Sept. 11, 2001, should serve America and its allies in the face of a possible growing threat.

As recently reported, dozens of seasoned militant fighters, including some midlevel planners, have traveled to Syria from Pakistan in recent months. American intelligence and counterterrorism officials have indicated that this could be an effort to lay the foundation for future strikes against Europe and the United States.

Among those voicing some concern is CIA Director John O. Brennan. He expressed to a House panel worry about the use of Syrian territory by al-Qaida to recruit individuals.

In addition, he cautioned that the organization may be looking to develop capabilities to carry out attacks inside Syria and use it as a launching pad to take aggressive action against the West.

Brennan said he believes these extremists are part of a group of al-Qaida operatives in Pakistan whose numbers have been depleted in recent years by American drone strikes. Still, they have a wide range of battlefield skills that make their gathering troubling, despite the fact that they are not believed to have experience in launching attacks in the West.

Syria reportedly offers terrorists cover from drone strikes in Afghanistan and Pakistan and access to American and European Muslims who have traveled to the country to fight and could be persuaded to carry out attacks on their home turfs.

Every theory has its naysayers, and this one is no different. There are those in the intelligence arena who believe there is no great, looming threat. Either way, any growing concern among top-level intelligence directors should be taken seriously. America and its allies must consider strategies ahead of any attempt at striking out, even if the likelihood of that succeeding is only marginal.

The intelligence community is basing its presumptions on electronic intercepts, informers and social media posts that conclude al-Qaida’s senior leadership in Pakistan, including Ayman al-Zawahri, is planning for the long term. The idea would be to identify, recruit and train Westerners who could move about freely.

This is certainly not out of the range of possibility. Whether or not the latest movements amount only to a patchwork of operatives focused on attacking Syrian government troops and rival rebel factions really isn’t the point.

Unlike 2001, at least American security officials are watching. The Obama administration and its allies must ensure that Syria does not become the next Afghanistan.

It should be noted that Homeland Security Secretary Jeh C. Johnson said in his first speech, “Syria has become a matter of homeland security.”

The government of Syrian President Bashar al-Assad has drawn foreign fighters like moths to a flame. Terrorist groups with an agenda of launching mass destruction from a war-torn country should not be allowed even the semblance of a safe haven.