WASHINGTON — In the months after the Sept. 11, 2001, terrorist attacks, Western officials worried about additional attacks, possibly using weapons of mass destruction, that could kill thousands. The United States invested heavily in biological and nuclear detectors and other high-tech gear. Jihadist terrorism seemed a menace that would unnerve entire countries and might last for a generation.

More than a decade and a half later, the threat and fear have proved real and lasting. But the death tolls in individual attacks in the West have remained relatively modest, partly because the assailants have learned that they do not need anthrax or dirty bombs to disrupt capitals, terrify tourists, rivet the attention of governments and impress potential recruits.

All they need is a gun, or, if that is too hard to acquire, a truck and a knife. And with simple preparation, such plotting, encouraged and sometimes directed by the Islamic State, is difficult to detect even with robust intelligence and law enforcement surveillance.

In the aftermath of the van-and-knife assault that left seven people dead in London on Saturday night — the third deadly attack in three months in Britain — it is hard to remember that years ago many experts predicted slaughter on a far larger scale.