If Americans had much to mourn on the 10th anniversary of 9/11, they also had reason to celebrate the end of Osama bin Laden and others who were among the terrorist plotters.

The last major al-Qaida operation was in 2005, when al-Qaida struck the London subways, but the “asymmetric” nature of terrorism is that small cells with small-scale weapons can leverage their killing power.

Another reason to worry: terrorists with more large-scale weapons.

“It is this specter of the lone fanatic or small group armed with the world’s most devastating weapons that keeps experts up at night,” says Amy Zegart, of the Hoover Institution at Stanford University.

In the world today, there is enough nuclear material to build 120,000 weapons, she wrote in The Los Angeles Times. “As long as fissile material is poorly stored and rogue states like Iran and North Korea continue their illicit weapons programs, nuclear terrorism remains a haunting possibility.”

While the worst prospects may not occur, she emphasized the need for not only vigilance abroad but more support for the FBI and its domestic intelligence operations.

Those are a key line of defense against a radicalized American willing to turn on his country and work with terrorists.

It is a timely warning in the wake of the 9/11 commemorations.