Audrey Kurth Cronin, a professor at the National War College in Washington, cites the arcs of previous violent extremist groups, from the Russian People’s Will to the Irish Republican Army, that she studied for her new book, “How Terrorism Ends.”

“I think Al Qaeda is in the process of imploding,” she said. “This is not necessarily the end. But the trends are in a good direction.”…

In addition, Al Qaeda, for all its talk of global religious war, offered no practical solutions for local problems: unemployment, poverty, official corruption and poor education. “People realized bin Laden didn’t have anything to offer,” Dr. Mandaville said…

Dr. Sageman said the United States should approach the threat not with hysteria, but with a careful analysis of the motives and patterns of people drawn into violent plotting.

“Terrorism,” he added, “is here to stay.”