Although the history of terrorist atrocities may stretch back through the decades, it was that sunny September day at the start of a new millennium that seemed to define a new era in terrorism. Fifteen years on, this seminal event continues to reverberate around the world. But the terrorist threat we faced then is much different than what we face today.

After the attacks of Sept. 11, 2001, much of the U.S. counterterrorism focus was on al-Qaeda. Under bin Laden it led the salafi-jihadi movement, orchestrating attacks globally. Today, its role as the vanguard of this movement has now been usurped by the emergence of the group calling itself Islamic State (IS), which has taken terrorism to a level of barbarism that al-Qaeda under bin Laden never reached.