Last week, federal jurors in Brooklyn heard tapes from an undercover informant in what one prosecutor called one of “the most chilling plots imaginable,” a 2007 Islamist plan to detonate underground fuel tanks at JFK International Airport.

On the tapes, defendant Russell Defreitas promised “high-tech,” “ninja-style” tactics that included releasing rats in the main terminal to distract security. “We got to come up with supernatural things,” he told the informant.

Despite his bluster, Defreitas seemed unaware of the technical difficulties involved in igniting hardened underground pipelines, and he never secured explosives.

The JFK plotters’ trial follows May’s attempted Times Square bombing, in which Faisal Shahzad — trained in explosives at an al Qaeda camp in Pakistan — failed to set off a bomb made of gas cans, propane tanks, fireworks and nonflammable fertilizer.

You ever get the feeling that some of these guys aren’t the sharpest scimitars in the shed?

If so, you’re not alone. The notion of “savvy and sophisticated” Islamist supervillains is “wildly off the mark,” Brookings’ Daniel Byman and Christine Fair write in Atlantic magazine.

Many Afghan suicide bombers “never even make it out of their training camp,” thanks to the jihadi tradition of the pre-martyrdom “manly embrace”: “the pressure from these group hugs triggers the explosives in suicide vests.” (Theological question: Do you get fewer virgins for an own-goal?)

On the American home front, al Qaeda and its sympathizers often don’t look much brighter:

  • In 2006, an FBI sting rolled up the “Liberty City Seven,” whose ringleader, the Washington Post reported, “wanted to blow up the Sears Tower in Chicago, which would then fall into a nearby prison, freeing Muslim prisoners who would become the core of his Moorish army. With them, he would establish his own country.” Sounds like a plan!
  • 2007 saw the arrest of six Islamists who planned to launch an armed attack on New Jersey’s Fort Dix, but were rounded up after they “asked a store clerk to copy a video of them firing assault weapons and screaming about jihad.”
  • In 2003, Khalid Sheikh Mohammed associate Iyman Faris went to jail on charges involving a plan to topple the Brooklyn Bridge by severing its suspension cables with a blowtorch.
  • The 2005 Jose Padilla indictment revealed that some Islamic terrorists haven’t quite mastered speaking in code. One of Padilla’s co-defendants insisted he was just talking about sporting goods on the surveillance tapes, but couldn’t explain why he’d asked his co-conspirator if he had enough “soccer equipment” to “launch an attack on the enemy.”

Lest you think I’m just cherry-picking particularly incompetent jihadis, recall that the Bush administration once considered Padilla, an American citizen, too dangerous for a civilian trial, and cited Faris’ capture as the crown jewel of successes with its warrantless wiretapping program.

The fact that many terrorists are morons doesn’t mean all are, and even morons get lucky sometimes, so vigilance remains essential.

But the myth that al Qaeda is 100 feet tall has fed dramatic government growth since 9/11. The Washington Post’s new series on “Top Secret America” shows that D.C. has erected vast pyramids in the name of homeland security, with some 1,200 agencies and 850,000 people trolling through e‑mail and clear-cutting forests to produce mounds of useless, redundant intelligence reports.

We’ve given al Qaeda power over us they don’t deserve. When we recognize that they’re often inept and clownish, we weaken their ability to sow terror. For the sake of our liberty and security, it’s prudent and patriotic to allow an occasional smirk to cross your stiff upper lip.