The Al Qaeda leader was reportedly killed in Afghanistan by a U.S. drone strike.

In 2002, when I profiled Ayman al-Zawahiri for The New Yorker, he was called “the man behind bin Laden.” But since bin Laden was killed by American special forces, in 2011, Zawahiri has been Al Qaeda’s leader. Zawahiri and bin Laden were very different men, not friends but allies, using each other for the skills and resources they could each provide. Al Qaeda would not have survived without the dynamic they created together.

Zawahiri, reportedly killed in Afghanistan by a U.S. drone strike over the weekend, was a doctor—a highly educated professional who chose to devote himself to violent revolution. He formed his first cell, to overthrow the Egyptian government, when he was fifteen years old. In Al Qaeda, he provided the direction, and bin Laden supplied the money. Zawahiri was always in the background, and many people who studied Al Qaeda thought that bin Laden’s death would bring the curtain down on their creation.

And yet, after bin Laden’s death, Zawahiri held the organization together. Under his stewardship, the terror group grew from four hundred or so men on 9/11 to perhaps forty thousand today in Al Qaeda proper and its affiliates, which range from Morocco to India. Although Al Qaeda never had another attack comparable to 9/11, its intentions haven’t changed, its membership has grown, and, with the retreat by the U.S. and its allies from Afghanistan, Al Qaeda has regained its training ground. It is once again a force to be reckoned with. This is due in large part to Zawahiri.