“This is not altogether surprising because it is difficult. We do not really know what works. I retired nearly eight years ago. I know that a great deal of effort has gone into thinking about how to counter this toxic and murderous ideology. I believe that we must have a better understanding of the roots of terrorism than we used to, and a better understanding of how to divert people—particularly vulnerable young people who have, in some cases, been groomed and exploited—from their path.